I'm glad someone asked, I was wondering myself.

I hadn't noticed the Switch Application option in most menus for the first while, but found by trying it to view what was open rather than for actually switching apps, that there were several applications open that one might wonder why they are open. Some might seem obvious that they need to run in the background, like Messages, Phone or Home... but others Browser, PTT and BB Messenger -- although I can see them needing to be on standby, I can't close if I wanted to.

In my case, I have 8 open apps after a clean boot:

Pocketday
Blackberry Alerts
Home Screen
Blackberry Messenger
Browser
Push to Talk
Phone
Messages

Of the ones listed above, I can't see any reason for BB Messenger or PtT to be open (in my case), although I would guess Browser needs to be open along with Messages and Phone to monitor calls, emails, sms, etc. And Home Screen to... go home. Pocketday and BB Alerts is fine, of course as I know why those must be there.

Before I became "aware" of apps that were open, it was alarming to find which ones that I escaped out of instead of closing were still actually open and presumably using resources.