Just quickly, but open calendar is means that I can view someone else's calendar within the standard pilot website. If they block their calendar there then I have to access an old DOS system to view their calendar and it takes about 5-10 minutes depending on how proficient you are.
As to why. There are many reasons. If I want to try and work a swap, the fastest way to to look at the bid to see who has the trip I want and then where he lives to see if he has a trip to/from his home. But you need to check his calender to see if he still has the trip. Does he fly that trip all the time. Easier to do all from the calendar page. If the trip looks like "His type" of trip then, I don't usually bother with the swap request and move to the next guy.
If I'm dropping a trip I look to see who might want the trip and what are they doing now. Do I have a better deal for them. This mostly works once you know the guys that fly similar trips to you.
If I'm flying with a guy, I might want to see what he normally does. Does he fly this type of international trip frequently? Not that most people check very often, but when you want to check and you cannot, it is a hassle.
On reserve you can track reserve assignments easier and see if how your day/week might flow out for you.
I'm sure there are tons more but a few quick reasons I looked over the last year.