Great question — answering as the UphillTech team:
It's part of what Philip said and part for another reason. Club's want their players to feel comfy when they are not playing and you'll find a large majority of the seats have internal warming. However, one manager got rid of all the comfy seats, his reason, which makes sense to me is that the bench isn't a place where a player should want to be. The other part of the reason and I guess this is more down to the type of seats they use is sponsorship and the money that comes with it. You'll find that a large majority of football clubs are sponsored by car company's. So you'll see that Man Utd will use Audi seats for example, in football they'll try to make money from anything.
Full answer by Adam Cormack: https://sports.stackexchange.com/a/9201 (CC BY-SA 3.0).
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