You'd use something like Pendo, Mixpanel, Heap or whatever as a client-side tracking library for a web application.

...If it's your own web application, then why can't you do server-side tracking?

And if it's not your own web application, why should you be allowed to track it?

Because you want to track individuals, and to do that, you need consent, probably acquired by a popup of some sort.

Unless I am misunderstanding things rather deeply, I do not believe you need consent to track individuals' activity in your own app, with server-side logging.

You need consent to set cookies that are not relevant to the (user-side) functioning of your app/website—and since that's the only way for you to track users outside your own site (an oversimplification, but close enough), naturally you need it to track them beyond there.

> Unless I am misunderstanding things rather deeply, I do not believe you need consent to track individuals' activity in your own app, with server-side logging.

You would require consent for this as far as I understand.