Clubs always bring 2-3 extra players outside of the 18 because it's not unknown for players to get injured warming up and you don't want to end up with a depleted bench from the very start. The rest of the squad generally then gets complementary tickets to the games - that's more common practice than a rule though. I suspect in some places it's more usual than others - I wouldn't be surprised to hear that in the PL all non-injured players not selected "to travel", as they say, are expected to attend every game except in exception circumstances according to their contract, whereas I'm sure that other places are a lot more casual about it.