There is a tracking system for many states so you can see if your ballot was counted, as evident by Wemay’s post earlier. Combine this with available voter party registration databases, you have the ability to say XX% Party ZZZZ registered voters have voted early. This gives parties enough of a rough estimate to know how many more votes they have to manufacture, harvest, or invalidate.
If you don’t think voter fraud is a thing, you are sadly mistaken. Election Day voting is the safest way to ensure a fair election.