I find the debates useful when one of the candidates seems too old, overwhelmed or just plain out of it to be president.
The first1984 debate between Mondale and Reagan was useful because it was a good predicter of Reagan's coming problems with Alzheimer's. The 2nd 1984 debate was useful because it showed what Reagan was like when he was his normal self. Reagan's 2nd term was a combo of both versions. Reagan had his good days and he had his not-so-good days throughout his 2nd term.
The first (and only) 2024 debate between Trump and Biden was useful because everybody could clearly see how much Biden had deteriated. Democrats clearly saw it as a sign they had to move quickly and replace Biden. The following debate between Harris and Trump was useful just to show that Harris was not an old, confused, man.