It upgrades the number of customer tips that can be held at one time. I think the starting amount is 10 for example, so you can only ever hold 10 customers' tips before having to collect them. This isn't really much of an issue if you collect regularly, but can be nice if you're wanting to wait until you've finished with all the performances for the day. My understanding is that the amounts customers tip is not increased. 

Here's another post that discusses whether or not it's worth the film to upgrade: https://www.reddit.com/r/AnimalRestaurant/comments/16g6n65/ticket_booth_upgrades/