Usually the tank being dry for awhile causes parts of the silicone to fail which causes leaks. Especially if its an older tank. That is when you want to reseal it. And silicone won't bond with itself so you have to replace it the whole panel usually...some people just redo all the silicone.
Whenever you buy a used tank if it hasn't been setup or held water for over a year, its good to leak test it outside in case the silicone is bad in some spots.
Most of the time new tanks are water tight, but it doesn't hurt to check them too.