What importance can the popular vote serve despite giving the electoral college the actual decision of a presidential election?

None ,States run the elections

In a US presidential election, it doesn't and it never has done. The constitution is perfectly clear - the President is elected by electors chosen by the states, and it's up to each state how it chooses the electors. There isn't any requirement for the people to vote at all.

OK, since 1868, all the states have allowed the people to vote and chosen their electors that way. So it's possible to find out what the popular vote says and whether it would have produced the same president, and people like to talk about that. But that's all completely irrelevant speculation.

It has and never had any effect. it primarily is used by historians or political strategists.

It shows that most people hated Donald Dump from the start, and the number just keeps growing.

Nothing at all