There are times when someone may choose to use a white base or a clear base when printing on a white tee.
I think you could break this down into two reasons.
1, If the halftones are high LPI like 65lpi, and you use a clear base or a white ink, (I do prefer a white ink),
It helps soften the dots and seemingly melts the dots together (a tiny bit more). This help the image seem less dotty.
2, The top inks seem to work better (blend) better, when they have something to bite into rather than airy cotton. I may alter this base a little and remove some content at times. A but if a modified base for this occasion.
When I do this it’s always on a high mesh like 230 minimum and more so a 300 mesh. We don’t want too much.
Like Sbrem has said, many can use the top white. This can help with any pastel colors. Some can consider this a crush white if they set it down under some colors.