Well, there are a multitude of reasons that you might want Rylai's Crystal Scepter. The main reason to get it is because it doesn't just increase the slow on your E—it instantly turns a single cast into a root, and a double cast into a stun. It gives you the ability to land Neeko-esque CC fairly reliably, which really helps to set up kills. The slow also helps you land your ultimate more easily.

Rylai's and Glacial Augment might be a little overkill, but it does mean you have even more constant CC, and with CC comes the mental advantage. I'm not even kidding. If you're trying to walk into river to get vision on baron, and a seraphine's single E roots you and slows you to a snail's pace, you're probably going to run.