I've donated to FTN and other places for many reasons both good and bad.
The good ( or at least less bad) :
The simple irrational joy that you get in giving .Sometimes the simple fact is that it is better to give than to receive.
Since I use the place and they don't run on air I felt it was the right thing to do.
They were crying low on funds and although maybe stupid I'm not entirely heartless.
Not directly to the tracker per se but when some charity was involved.
The neutral :
To get a

invite(s) for someone I either knew well or maybe only casually but thought "deserved" one ( I did say I might be stupid)
Although not applicable to FTN -for upload credit /userclass/immunity from pruning etc.I'm not especially proud of the fact but since the tracker itself makes it an option it's (hopefully)not intrinsically "bad".
The bad:
Sadly for simply "status".To have a star or whatever the associated site might give you and so consequentially been seen as being "good" even though in specific cases because of the reasoning behind it that would be far from true.
That said between all those reasons I don't think rightly or wrongly that there are many trackers that I belong/have belonged to that I haven't given at least a little money .
Or course the same could apparently be said about people who buy invites to simply resell them on Ebay so that fact alone hardly makes me better than anyone else .