Voting early is a gift to a campaign.   Once you vote, the campaign can remove you from their outreach pool.  They can then focus on reaching those who are undecided or less inclined to vote.

Not only is Harris's lead in early voting a good sign for her, but it's a very real tactical advantage. 

This is also yet another reason why Trump not liking early voting is a sign of his poor instincts and bad decision-making.