Because it builds momentum, increases the perception of the power of his endorsement. So, when the endorsement grows in perceived power, at some or several points its ‘power’ will tilt the balance somewhere critical. In politics perception is almost everything!
What power of his endorsement? McCarthy is going to win this overwhelmingly with or without Trump's endorsement. If anything it makes Trump look a bit pathetic. Like he's jumping on the McCarthy bandwagon instead of the other way around.