[Mobhack]

There is no difference in hitting between using the left mouse or the T key.

Just that with the left mouse key, you may select something other than the intended target if there are others in the same hex. Plus, running through the T key selection lets you have a "gunners guestimate" of what shots seem to be better prospects - that other tank over there may be 30% whereas the one you were thinking of comes up as say 19%.