Getting extra chronicle sheets - No necessary. When I speak to most of my friends who GM, after a while, the credits just stack up somewhere. I have couple of GM babies who are "clone" replacement, they will be used in case one of my favorite characters die. I just rather play my characters, they are fun that is why I made them.
GM more than once - For me, GMing a scenario a few times is a benefit for all those involved. I like to get the exact monsters and maps for the scenario that I GM. Getting to use it a few times such make it more cost effective. Spending extra time to prep make a game better. I found that players always surprise me On how they handle a situation and each time I run the same scenario, I can handle the unexpected better. Run the same scenario more times make a GM specialized in that scenario.
Award and recognition - maybe not necessary, but if someone who run the same scenario five times, she can be called "veteran GM for 07-01", if they ran it 10 times, they can be called "master GM for 07-01"
Boons - if you become a Veteran GM for a few scenarios, you get a boon. This way, Paizo can just come up with 1 boon sheet and it should covered it.