Iis – Identify which application pool is using CPU / Memory

Yes, many:

• AppPools can run as different identities, so you can restrict permissions this way.
• You can assign a different identity to each app pool so that when you run task manager, you know which w3wp.exe is which.
• You can recycle/restart one app pool without affecting the sites that are running in different app pools.
• If you have a website that has a memory leak or generally misbehaves, you can place it in an app pool so it doesn't affect the other web sites
• If you have a website that is very CPU-intensive (like resizing photos, for instance), you can place it in its own app pool and throttle its CPU utilization
• If you have multiple websites that each have their own SQL database, you can use active directory authentication instead of storing usernames/passwords in web.config.

I know this is an old question, but it came up under related questions so I thought I'd chime in for those who might come across this at a later date. Scott Forsyth recommends against running web gardens because of the many challenges they raise. He has a video on the subject which he intros with:

This week’s lesson discusses why it’s rarely worth using Web Gardens and why using them is often detrimental to your server’s performance.

The biggest caveat is the implications for session state, but there are other challenges as well.