To be notified of updates. Why in preference to email? Web feeds are typically presented as information streams while email is presented as interpersonal dialogue. Organizing your information streams into an aggregator is clarifying.

> Why?

Cannot answer for parent post, but can share personal perspective. I love Atom / RSS because it allows me to aggregate all the information that I care about in one place. It greatly improves the signal to noise ratio and spares my communication channels for direct human interactions.