In the older Spider-man comics (although it pops again every now and then) he is almost always drawn with webbing in his arm pits as part of his costume.
Have these things been given any function in Spidey canon?
The webs were something Ditko liked and continued to do on and off for years. They were drawn so large, for a time they looked like wings and even other artists questioned if they could have another purpose.
When John Romita started drawing Spider-Man after Ditko left he drew the webbing smaller and smaller and eventually left them off, altogether. If you have never had the pleasure of reading Amazing Fantasy #15, first tale of Spider-Man, no fears True Believer, you can read it here.
When the Amazing Spider-Man #1 came out they included a list of things Spider-Man was able to create with his webbing. This included creating a parachute!
I previously answered a question similar to this as to whether Spidey has ever used the under arm webbing for gliding. Most of the same answers apply here: in most cases in the comic, the under arm webs are there as a purely stylistic choice for more dramatic affect; they are simply too small to be used for "Gliding" purposes, as they are. However, they have been used for gliding in some instances.
A more specific answer to This question, however, was undertaken by Scott Nisewander at Nerdsync; I think you will find it very helpful and interesting.: