Bumps in the road will feel a little harsher, and could damage the suspension a little more easily.
Steering will feel a little lighter.
Gas mileage will improve slightly.
Dry traction may decrease slightly.
Resistance to hydroplaning will improve slightly.
Many people (including mechanics for customers) add a few extra pounds to the tires to make up for pressure loss over time, especially since most drivers don’t check their tires often enough.
Since tire pressure changes with temperature, pressure should be checked when the tires are at their coldest.
I check on a cool morning after the car sits overnight.