Because labels are configured manually, it is possible for assigned labels to get out of sync with the reality in the slave machines. This kind of error can be typically only detected by having some builds fail, or worse, it can go completely unnoticed.
By checking this checkbox, Hudson will programmatically perform check(s) whenever a node becomes online that claims to have this label. The actual check logic is extensible — you can write/install plugins to expand the types of checks available.
When one of the configured checks fail, the node will not become online. The administrator will have to go in and reconfigure the label assignment before the said node can be used by Hudson for builds.