Here's a typical example of how the jumpseat reservation works. I am junior, so I ride the bid sheet a lot, so i don't always know my schedule more than 2 days in advance. This week I'll be flying out on Wed for a morning trip on Thurs. I have a 2 leg, cross country commute. I checked flights Mon night, 36 hours out. For the first leg there are 7 open seats. I got the primary. The second leg is PHX-PHL, a route that's typically full. The first flight after I get in is full, neither jumpseat reserved. It's a 29 min turn, so I decide to go with the next flight where I am the Alternate and if I get in early I'll try for the earlier flight.
I'm junior, booked my commute 36 hours out and got my first choice on both flights, one being a commuter heavy route. Stress level for my commute is pretty much zero. This is pretty typical.