string tree

[nofi]

this may sound dumb but what is the purpose of this thing. some basses have it, some don't. i suspect it is some sort of geometry/string break on the nut kind of thing. but my p bass mistake plays and sounds the same with or without it. please explain.

[hollowbody]

The string tree puts downward pressure on the strings as it passes over the nut.  If you don't have a good angle over the nut you'll get buzzing and rattles. 

If you have enough windings on your machine heads you can usually create a decent angle and the string won't buzz.  That's why I usually shoot for about four windings on each post when stringing a Fender.

[Dave W]

In addition to buzzing and rattling, too low a break angle can also cause resonances at the nut that affect your tuning.

If you're not having these problems, then there's nothing to worry about.