Question

why gloucose gives the brain and muscles a quick bursst of energy ?

Answer 1

While ketones can substitute for a portion of the brain's energy needs, it cannot substitute completely: blood glucose levels below 2--3mM somewhere causes serious neurological problems and can lead to unconsciousness.

Other body tissues are not strictly dependent on glucose, but can oxidize amino acids as well, which is a good backup solution since there is always lots of protein around. But not the brain.

In terms of evolution, this strict glucose dependence must be a major drawback --- falling unconscious just because you don't get enough sugar is probably a bad thing out in the wilderness, when the lions are after you ... So there must be a very important reason.