Honestly, I had no idea there would be so many tip calculators on the App Store when I started working on TipOver. On the other hand, I wasn't that far along on it's development when the App Store launched and the tip calculator competitive landscape came into focus and yet I still chose to continue with it. There were a couple of reasons for this.
First, I saw it as an opportunity to cut my teeth on iPhone programming. I wanted a real project that (I thought) would be relatively simple to create while learning a development environment I was unfamiliar with. I've only been a Mac user for about a year and just started looking into Mac programming a few months ago. Outside of the great Cocoa Programming book, developing TipOver was my first real experience with the Apple development stack (XCode, Cocoa, Interface Builder, etc.). It turned out to be a bit of a challenge since, at the time, the Draconian iPhone NDA was still in place and there was no good (legal) place to go to get my questions answered. Nevertheless, I managed to get through it and am happy with the result.
The main reason, though, that I decided to continue creating another tip calculator was the fact that, out of the countless others on the store, I couldn't find any that had the one feature I wanted. While most of the decent tip calculators have a rounding function, none (that I found, anyway) had the ability to round up to the nearest total that can be equally split by the number of people in the party. I am a bit obsessed with simplicity and find that this is the most effective and fair way to deal with splitting the check with a group of people.
So, there you have my rationalizations for crowding the App Store with yet another tip calculator. I certainly have found it very useful and the feedback I have received so far has been very positive.