Tar is used to explore common crypts. Exploring such crypts allows you to get materials for crafting standard equipment. The higher the level of a crypt, the more tar you need to spend in order to complete the exploration and the more materials you will get in the end. Researching various crypts grants you the possibility to obtain two specific materials instead of one: the particular material depends on the crypt explored. However, note that this chance is never 100%!