Early voting does have disadvantages, especially in nomination contests where candidates may drop out between the printing of ballots and the casting of votes. But it also has advantages.
Stephen Wolf at Daily Kos:
As shown on the map at the top of this post (see here for a larger version), Colorado, Hawaii, Oregon, Utah, and Washington have already transitioned to universal voting by mail ahead of November's elections, and states such as California and Arizona have seen a majority of voters cast their ballots by mail in recent years. Furthermore, states such as Michigan, Pennsylvania, and Virginia have made it considerably easier to vote absentee by mail since the last election. But a large number of states still don’t even allow absentee mail voting without an excuse, let alone mail every registered voter a ballot as the default voting method.
Adopting universal voting by mail is imperative as the coronavirus threatens to become a pandemic. As governments urge citizens to avoid crowded public spaces and to vigilantly wash their hands, having millions of people stand in lines on Election Day and use the same voting machines or even the same pens to fill out paper ballots threatens public health and could dampen voter turnout. Furthermore, because poll workers are very disproportionately the same elderly Americans who are most at risk in a flu pandemic, states and localities may have trouble finding enough poll workers, and those who do show up will be at greater risk of catching or spreading the disease.
These problems should be largely avoidable with universal voting by mail, since it almost entirely eliminates in-person voting and the public gatherings that go with it. Furthermore, vote-by-mail is a cost-saving measure that frees up resources for other things because governments have to operate far fewer in-person polling places. It also has a long track record of increasing voter turnout in the states that have adopted it thanks to the ease of voting that way, especially with prepaid postage negating the need for voters to go to the post office.