Why would you need DRM for a web app with a subscription model?

To stop subscribers copying all the content and then unsubscribing. For services where most of the value is in the back catalogue this is a risk.

If that's all the value you're offering your users, DRM is the least of your problems. If it's that easy for users to exhaust the content they actually want, you're going to have problems with customer retention, not to mention customer acquisition.