When to use "unlock upgrade"

By disha

I see an unlock upgrade button in my Elgg install.
I read this http://reference.elgg.org/engine_2lib_2upgrade_8php.html but have no clear idea when this button needs to be clicked or what it does.

BTW, is there any prior way to know whether an upgrade makes any change to the database or is just a matter of changes in files or *both* ?