The person holding a place for you is also giving up the freedom to use it differently.

Saving a seat, a slot, an evening looks costless from outside. Whoever holds it has stopped being able to move, though. They cannot leave, cannot rearrange, cannot take the better offer that turned up at four. Your flexibility is being funded out of theirs, and it never appears anywhere as an expense.

When someone is holding something for you, arrive when you said or release it early enough to be useful. A message at the last minute is not the same as giving it back.