Question

In hilly region, road is constructed with a number of turns. Why?

Answer 1

The main reason is bad soil at that location.
Obstacles is another reason. There might be a temple, important structure, etc on the straight path.
Natural factors like Lake, hill etc will force the engineer to deviate the road if the budget is less, as filling lakes and breaking rocks is expensive.
In hilly region, gradient matter a lot. A low powered vehicle cannot climb a higher gradient.
Cost matter a lot.