A standard leg rest of the cushion type can be comfortable and useful for relieving your legs and feet but sometimes the height may be less than ideal for you.
Certain disabilities may make it more difficult for you to be in a comfortable position, so adjustable aids can often be the preference as you need to be able to get something positioned at an angle and height that is just right for you.
As an aid, often people will be looking for a leg rest with less soft cushioning, still padded but the ability to adjust the height is important, and because a user may not always require the same height, that also gives the person the need to adjust the leg rest as required.
On castor wheels they are easy to move around into position when you need to use the leg rest, if you find that one day is completely different from the next for you, with regards to the type of elevation that you need then you have the easy option of adjusting the height, rather than being stuck with the same height and not getting the desired relief for your legs and feet.
It also means that it wont matter what type of chair that you are seated in because the adjustable leg rest can be suited to any position and height that you are in at the time.
Being adjustable contributes to being comfortable.