Our different saddle models have different seat depths. The depth of the seat influences how the rider is supported in the saddle.
A deep seat usually supports the rider in a more upright and aligned position, and offers more support, especially in trot and canter. A deep seat has a higher cantle, which prevents the rider from sliding backwards in the saddle and getting behind the motion of the horse - especially when riding faster.
A less deep seat allows the rider to move backwards and forward in the saddle. It offers less support in the cantle and allows more changes in the position of the rider.
There is no general rule which seat depth is best for an individual rider. The main criterion is that the rider is comfortable and not restricted by the saddle. It is important to try different kinds of seats while actually on horseback, and find which type of seat suits the individual conformation and preferences of the rider best.
A deeper seat can help a rider who tends to sit in a “chair seat”, with the legs in front of their center of gravity, sitting on the back of the buttocks instead of centered on the seat bones. This common fault badly affects the horse’s balance and prevents the rider from giving correct aids.
Although a good saddle can improve the position and balance of a rider a lot, the rider must not be forced into a fixed position by the saddle. That always leads to stiff and tense riding.