Not the OP, but:

1. General awareness about what's on your plate. You don't need a reminder, you just need to know that you have meeting X at Y hours, or Z appointment on this date.

2. The difference is that you check email/IMs on your terms, which means that you decide when you want to take a break from whatever you're doing to look at and respond to anything in your inbox. If you have notifications on, your focus will keep getting taken away from whatever you're doing. Depending on what you're doing, this may or may not be important to you.