It's not so much that as that the coalitions and eventual parties wanted to hold both seats, so they ran multiple candidates with the assumption that one would be president and the other vice president. The electors would then structure their votes to ensure that the correct person was elected to each position. However, with the difficulties in long-distance communication at the time, this was prone to error. In 1800, this almost led to the candidate for vice president being elected as president.
After that, they realized that it didn't make sense to use one slate of candidates for both positions, so they separated out the ballot into president and vice president. That's essentially how the elections had been running up to that point (particularly because they always had two votes to cast), but it was to easy to make a mistake. Both before and after the amendment, there was a presidential candidate with a running mate vying for the vice presidency.
At the national level, that's true. The candidates are usually quite distinct and very well known, so holding a particular endorsement is unlikely to change anything.
However, I do find them useful in local elections. In those, the candidates are usually (but not always) pretty closely aligned, so it's hard to make a decision based off of what their campaign is promising. They also frequently involve candidates that are fairly new to politics, so it can be difficult to learn more about their past outside of what their campaign puts forth. So I'll usually learn something worthwhile from an endorsement that can help me make a decision. I also have a good opinion of some of the local magazines that make me more willing to trust their recommendations.