It enables to recover filesystems consistency before they are used

To speed the process, the filesystem journal can be executed

When the journal fails, which is quite uncommon, filesystem check can nonetheless recover consistency

It is always needed for readonly filesystems

It is needed at system boot for all filesystems

It takes a quite long amount of time to execute for large filesystems

It is done at system boot for some filesystems when they have been left in an inconsistent state beforehand