Lately, it seems to me at least, woodwinds have become a mystery instrument in the sense that -- it seems like lots of composers are scared to write for them. I'm hoping we can demystify what are -- I think -- the most important coloring instrument in the orchestra.
What are woodwinds?
They're peculiar, to be sure. The brass all share a common tonality, with the horns juuust skirting the line. The strings are obviously all the same general timbre. Percussion varies but generally rely on non-musical tonality or are some kind of idiophone.
So what's with woodwinds?
You have flutes which range from well-rounded to ear piercing but overall soft and silky. But next down the line you have an oboe and they are about as far from the flute as you can get. They're nasal and cut through the stage with ease. Oboe's tend to have a gentle touch as soloists with great inflections when played by a great player. Now they don't have the lightness of the flute but boy do they blend well together! Moving on we have oboe's kissing cousin, the english horn (or cor anglais if you want). Essentially, the english horn is a deeper oboe but there's a certain sorrowful quality to these oft un-utilized instruments that makes them a bit more akin to their ancestor, the duduk. Clarinet's are next and they're the only single-reed instrument we have. Much closer to the flute than we've seen so far, especially in the upper mid-range. Once you get up higher in pitch you start to get an almost squealing quality to it. The low range of the clarinet is fantastic and dark with a warm earthy buzz. There are other types of clarinets each with their own quirks (Eb clarinet is a common guest star for example) but the one that really needs to be talked about is the bass clarinet. So where the Eb variety really shares a lot in common -- sonically -- with it's siblings, the bass clarinet does not. With textures ranging from brash and woody in the low end to it's thick and rich highs the bass variety is very hard to wrangle in. It doesn't take much to go from a soft and mellow line to suddenly overtaking a trio so it's best to stay conservative with them unless you know for sure that line needs to pop! The bassoons are maybe the easiest member of the woodwind clan to write for because: they just fit everywhere. It's so hard to mess up a bassoon part I'd almost say it's impossible (but I believe I can do it)! They blend with every member of the orchestra so incredibly well due to their warm-yet-nasal sound. They're very similar to the oboe and english horn because of the double reed but share a lot in common with clarinets and flutes because of their full-bodiedness.
The remarkable thing about the woodwinds is that while they all sound so different, they can be written as an ensemble with relative ease, if you're careful. There are some general considerations to take into account like your voicings, a chord that sounds great on a piano will, 9 times out of 10, not translate to woodwinds exactly. Because they all have such unique characteristics inevitably the chord will be completely unbalanced. Great care needs to be taken to get the desired result, even more-so when working with a classical sized woodwind ensemble!
Often times woodwinds should be arranged in their standard order.
This means that bass clef instruments are almost always in the low end and vice versa. Whereas in string or brass writing you can easily freeflow your chord work in woodwinds, while not impossible, it's much harder to make work. There is an old concept in orchestration that still rings true regarding sound as color. You don't apply a certain color to each instrument but your considerations should be that of a painter. A composer is blending colors and creating a work of art out of the abstract. In the same vein, woodwinds are much like the color spectrum. If you've ever seen a chart of the wavelengths of visible light, or if you've used Photoshop, you have most likely seen a representation of the color spectrum. It shows all visible colors as they blend from darkest to lightest -- it's a perfectly smooth and gradual throughout. This is the woodwind section. Woodwinds are a perfect blend from dark to light. The Bassoon blends to the Bass Clarinet which blends to the Clarinet which blends to the English Horn which blends to the Oboe which blends to the Flute which blends to the Piccolo. If that sounds contradictory from earlier, it's because we're talking about the gradual blend from one to the next vs the general sonic qualities. They're an interpretation of color as sound -- all in one section! This doesn't mean they have to be in perfect order, painters don't keep everything laid out in a row either, they mix and match and create new colors. Understanding where they function within each other is how you start. Having a fundamental understanding of a woodwinds placement is key to breaking down the enigma of where they fit in (hint: the answer is everywhere).
The Violins' melody needs to cut through? Double with Flutes or Oboes or Flutes and Oboes (or Clarinets).
The Cellos need to stand out or have a rich(er) quality? Double with Bassoons.
The Trumpets need that "silvery" quality? Double with Flute and Piccolo an octave and two octaves above (respectively).
Do you need chord work/movement behind the strings/brass/choir/whatever that doesn't get in the way? I think you know the answer.
Woodwinds are amazing at filling the space without anyone realizing it. It's like the bass guitar in a band, loads of people don't know it's there but if you removed it the mix would feel empty (cough cough...And Justice For All cough cough). They are essential in creating mobility behind melodic lines via chordal work or runs or ostinato. They're also criminally underutilized nowadays but, hopefully, we can change that. There are countless musical works I've heard from contemporaries where it's almost frustrating how they've been left out! And leaving them out has it's place as well but you should have a good reason for it! Don't just leave them out because of a lack of understanding. You have to learn how to use your tools.
TL;DR: If you're asking yourself if you need woodwinds in a piece the answer is probably yes, in some way.