Take Gmail. I want it easily accessible, but I don't need it running in the background as my phone will ping when I got a new mail.

When developing, I'll open a new window and keep some tabs relevant for what I'm working on. Frequently it involves me piecing together bits from different APIs or parts of APIs, so require switching between multiple tabs. Often I'll have to come back, and it may take a day or two. No need to keep the tabs running, I just want to quickly pick up where I left.

Stuff like that.