Depends on the school.
For pretty much every school, your PS, MCAT/GPA, ECs, and LORs in the primary will be the biggest focus. However, many schools take their secondary quite seriously, especially the ones that receive a bunch of apps. They want to make sure that you really want to go there. Your secondaries aren't expected to be as polished as your PS, but take the time and really cater it to the specific school.
Now, there are a lot of secondaries that are similar to each other, so you can certainly re-use essays or parts of essays. However, really put the time into the "why our school?" questions.