When you scale down a playermodel in the workshop editor to match the normal player size, it also changes the actual player size, also affecting size, eye height, movement, etc. However, in some cases, this isn’t wanted, particularly with playermodels using big heads, where the actual model is normal-sized, but the player size is extremely tiny.
In those kinds of cases, it would be better to be able to set player size independent of playermodel size, so you can have a model with a big head and small torso at normal player height, similar to playermodel behavior in games such as Virus.