The purpose, as I've seen(I've never done true formal agile but I think it influenced some of the people I worked) is that some businesspeople seem to like to see constant visible progress.

4 days is too long for them to wait, they get antsy and think nothing is happening if you can't actually show them something they can understand in a day or two.

I think it actually affects the code, everything has to be taped together tiny bits, because you can never do anything that doesn't show results fast, so you if you need a framework or an architecture or a plugin system, it's really hard.

But agile says that's fine because you're going to be doing first drafts and throwing away code anyway....