There is an achievement related to giving somebody some money in the game. Also I save my money because sometimes the vending machines in the game as you get higher will if they do the same as BL offer up some pretty unque items once in awhile. Plus it is just cool to have lots of money even if it is a video game.
Keep tipping Moxxi until you get your achievement and then the naughty cutscene. --- "After time adrift among open stars. Along tides of light and through shoals of dust. I will return to where I began." Keelah se'lai.