Ah yes, very important. One piece of advice - stick to them!
If you are entering a competition or submitting to a magazine and they have a word count, don't go over it. When I was studying for my Humanities Certificate with Open University all our assignments had word counts. In this instance tutors allowed us a 10% 'over-spill' but even then some people couldn't keep to it saying they couldn't write all they wanted in that set word count. That's where the technique of editing comes in - cut those useless words, the ramble, the line that says the same thing but just in a different way. I found it good practice and I only once or twice went over into the extra 10% by a matter of words a couple of times.
When you look at a piece its always possible to cut. Afterwards you can actually see/hear the difference.
It's tighter and more readable.
In poetry writing word count doesn't really crop up......it's all down to lines. The same rules applies You already know how many lines you are allowed so you aren't going to try writing some epic saga.
So, word counts are good editing practice and always stick to the rules!