It seems to be an inexpensive (yet somewhat confusing) way for folks to send you money without them knowing your address. Also without you knowing who they are (if they want that). And also by credit card, if they choose (and that's REAL tricky with Tip Jar, in my recent experience).
It's not a bad idea for your site, especially if you only need $200-$300 a year to really have it self-standing...