Let Grandpa tell you. Back in the twenties most cars lacked power enough to climb
steep hills in third gear. No automatic shifts then. And if you didn't downshift
into second gear about halfway up the hill, your engine was very likely to stall.

Sometimes going down very steep hills, it was also desirable to downshift into second
gear to have engine act as brake and avoid going too fast. I heard of trucks having
brakes fail, and with a hairpin turn at bottom of hill, going off road right through a house.