I know that raw food is indigestible than cooked. We sure may eat raw vegetables & fruits, but I can't say same about potatoes. We can't eat fresh meat and eggs because we can take infection.

We can eat food raw. Have you never heard of sashimi, or the Raw Foodism Movement? To answer your question though, humans started to cook meat many thousands of years ago for a couple of fundamental reasons. One is that meat is easier to chew when it is cooked (thereby causing us to expend far less precious energy), and another is that cooked meat is more easily digested than raw meat. Both of these factors allowed humans to save energy which was instead supplied to other bodily functions, and especially to the brain. The move towards cooked meat is therefore thought to be in large part responsible for the rewiring and rapid evolution of the human brain. Cooked meat also carries health benefits, as the possibility of infection was reduced given that the cooking process tended to kill most bacteria.