I carry my phone around a lot. I might lose my phone or it might get stolen.

I also don't want to be locked into a single application. Especially when testing a new software. I want to keep my data and not be bound to a single service.

For introspection/QS purposes it's also good to have the data in a way where I can analyse it further. For example I log all calls and all pomodoros I do to a Google calendar. Otherwise most of my data goes to Evernote.