It's not for lost + found, it's used for system reserved space.
By default 5% of the space of an ext partition is reserved for roots use only in case of emergencies, for example runaway processes filling up the disk with log files.
Having a reserved space means that the system would still be able to keep running in such situations.
5% used to be a reasonable amount back when hard drives were in the 10's-100's of MB size range, but in my opinion it is far too much nowadays with TB size data drives that the OS doesn't live on anyway.
Fortunately you can alter the amount of reserved space with the tune2fs command when the drive isn't mounted, for example...
would change the amount of reserved space to 1% (30GB).
sudo umount /dev/sda2
sudo tune2fs -m 1 /dev/sda2
If this is just a data drive you can quite safely get rid of this reserved space altogether by changing it to 0%.
You can check the current status of this and much more information about an ext partition by using the list option...
sudo tune2fs -l /dev/sda2