what is the advantage of an energy store in the form of tryglyceride rather than glycogen?

Answer 1

One advantage is that the amount of ATP produced by metabolizing triglycerides is much greater than the amount produced by metabolizing glycogen, on an ATP/gram or ATP/mole basis.

Second, glycogen is very highly hydrated, and so has a lot of water associated with it. Triglyceride is stored a pretty much pure droplets of lipid with no associated water.