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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.