I don't know the maths of it, but I'll have a couple of guesses.

1. Any altitude gained above the intended orbital altitude (that is, the climb from 200 miles to 270 miles) represents potential energy that can be cashed in for speed when the rocket dives back down to orbital altitude. My guess is that this potential energy is more cheaply gained when the rocket is going more slowly, and more efficiently spent when the rocket is going faster.

2. By climbing to the higher altitude the rocket is able to gain speed more efficiently by encountering less drag. Because drag increases at the square of the speed (or something like that), the high speed of a rocket means that even at orbital altitudes there are efficiencies to be gained by climbing even higher to where the atmosphere is even thinner.

How did I go?