Seriously, that’s it.
OK, so why? You have to ask? I use twitter on four different devices (2 desktops, my phone and my iPad) most days, and I simply can’t be bothered scrolling through tweets I’ve already read. TweetMarker takes that pain away from me by allowing my various devices to sync my last read tweet. It’s simple and it works.
I’m not saying I’ll definitely use a twitter client just because it supports TweetMarker, but I am saying I won’t use one that doesn’t.