The "boost" buttons are to do with the cars that have KERS systems. This is the Kinetic Energy Recovery System, basically a device that stores energy (typically a battery, but could be a flywheel) when braking. This energy can be used in the race via a button the steering wheel to provide about 80 bhp extra for up to 1.6 seconds a lap.
It was up to the teams if they wanted to run with KERS or not. Ferrari, McLaren, Renault and BMW have it, the others don't, and even those four don't run it all the time.
The problem with KERS is that it adds weight, and although this extra weight can be taken out of the ballast of the car, so the car overall doesn't weigh extra, it means that with less ballast there is less flexibility to distribute the weight exactly where you want it (i.e. for optimum handling).
The original idea was for it to help improve overtaking.