TB is a type of mycobacteria. When they run a culture on a sputum sample, they don't get specific bacterial identifications right away. If something comes up "gram positive" and "acid fast" (both of which refer to whether or not the bacteria shows up a certain way on the slide during the testing) then they might just want to eliminate TB right off the bat. I cultured a gram-positive, acid-fast bacteria and it took about 8 weeks to figure out specifically what it was, but TB was eliminated first because of the skin test they did.
It could just be a routine test, or it could be because something grew out on a culture and they wanted to be sure it wasn't TB because the test for it is pretty easy and relatively fast.
Good luck!