I think it's Apple's way of drumming up business. Imagine what happens when (note "when," not "if") a spill ruins the keyboard on your Mac. That's why I use a $5 timer rather than a $1000 computer for timing my darkroom activities.
This, like most things in the Apps store, isn't made by Apple, rather a third party (bamboo apps in this case).
And 'when' isn't really an issue with this, because it goes full screen, and you can start the timer with the play button on your remote so your computer never needs to be anywhere near danger.
Looks like a pretty good app. I'm going to have to give it a go.