Trailing edge devices such as flaps increase lift by raising lift coefficient at s given AoA. Leading edge devices such as slats increase lift by raising stall AoA.
The former is nice because it allows the nose to point lower and give the pilot a better view during landings and takeoffs, and increase tailstrike clearance. The latter does the opposite.
Why have both on aircraft, as is done today? Why not just use bigger flaps? Wouldn't that be safer and simpler?