Rare Tar is used to explore rare crypts. Exploring such crypts allows you to get gems to socket into any created equipment. Unlike common crypts, rare ones don’t give you any chance to get two specific gems instead of one.
Note that exploration of rare crypts requires not only preliminary relevant research in the Academy to be carried out, but also a specific key to open the crypt. You can get the key of a needed level by defeating Elven Strongholds of the same level.