They do matter:
The amount of influence the endorsement has, however, depends largely on how unexpected (or “credible,” to use the authors’ term) the endorsement was.
If a newspaper editorial page is perceived as liberal and endorses the Democratic candidate, for example, the endorsement has little effect, and likewise with a conservative editorial page endorsing a Republican candidate. Endorsements from newspapers seen as politically moderate, or those usually believed to be in the opposing political camp of their presidential pick, are much more influential. In other words, readers seek guidance from editorial pages on how they should vote, because they believe the writers are more informed about the issues at play. But readers still discount editorial page endorsements according to any bias they perceive in the newspaper’s political leanings.
The Dish 