Every n boot (n usually around 30) is scheduled a fsck at boot. It is more a legacy thing as ext4 is really robust. I remember checking my /home/lost+founds regularly in this time and often finding random stuff, music, documents where the i-node has been made orphan so that it doesn't belongs to any directory. Today, we have ntfs and ext4 with amazing journals which can replay most of the operation that has been made. So, when the Virtual-FS layer (the component of the kernel that map the bytes on your HDD to the logical file-tree in the exporer) detect an inconsistency it can replay and apply the correct state to the fs tree on the fly.

Now that we have robust disks and robust fs, I think that this fsck at boot setting could be relaxed. However, think of the people needing saving their data with an fsck because their HDD wasn't as robust as yours. It's not an easy choice to do. You can't even base your decision on the SMART metrics as the worst HDD, the one you want to check regularly, lie to/don't implement correctly SMART.