If your weight is firmly down onto the stirrup bars - and if you ride ENglish "forwards" or if you are a Western rider converted to English - then there might be a lot of weight on the bars, then the idea of these sitrrups is to cushion your foot, ankle and knee to the concussion coming up from the horse moving over hard surfaces - especially at the trot on tarmac.
In theory, if you are riding in a modern neo classical style then most of your weight is in the saddle and you are merely keeping your feet steady by resting them in the stirrups. If however you were taught to ride cross country with your weight over the horses centre of gravity then you will generally support more of your weight in the stirrups.
If you read the toes up/heels down and toes down/heels up debate maybe you'll get a better idea of what I am trying to say.
The other use for these stirrups irons is when jumping. The concussion coming up through the stirrups when the horse lands on its two front feet is tremendous both to the horse and the rider. Cushioned stirrup irons help absorb the concussion.
But usually those cushioned irons are expensive - so try them before you buy.