Flying a let's say A320 in Alternate Law with no auto trim would mean that you have to obviously trim airplane yourself. Which for example means if you want to ascend at 1000 FT/MIN then you need to have a slight pushback pressure on the sidestick all the time until reaching desired altitude. If auto trim exists and you want to let's say climb at the same rate of climb which we said would be 1000 FT/MIN then you need to apply slight pushback pressure on the sidesticks for a few seconds or shorter until climbing at 1000 FT/MIN then you need to release sidestick for it to come to the neutral (Center) position and let autotrim adjust it and let it keep climbing at a constant 1000 FT/MIN.
Or just trim to ascend at 1000 FT/MIN if in Alternate Law with no Auto Trim available.
Auto trim is good and does not require you to keep sidestick in a certain position when climbing or descending and keeps that descent/climb rate it stopped at when releasing sidestick.