At least according to polling at the time, Hillary Clinton "won" all three of the presidential debates in 2016. That, of course, didn't translate to the election win most people expected, and it begs the question - do the debates actually matter?
According to polling by Pew Research Center after each election since 1988, a fair share of voters do in fact say that the debates helped them to some degree in deciding which candidate to vote for. Notable exceptions to this are the Bush/Dukatis and Clinton/Dole/Perot elections, for which 48 and 41 percent found the debates useful, respectively.
In 2016, 63 percent of voters said the debates were helpful in the decision making process but, as in 2012, only 10 percent said that they actually made their decision during or directly after the debates. With an electorate seemingly even more polarized in 2020, that figure may well be even lower for this election.