No AMCAS secondaries yet, but all my TMDSAS secondaries are already submitted
To answer the previous poster, I think a long time ago you had to apply to every single medical school individually. Then as applicants began applying to more and more schools they decided to simplify life a little and create a centralized service that sent out a uniform application to all schools. This service handled all the "basic" stuff that was getting really repetitive to fill out with the old system--demographics, coursework, test scores, a general essay, and so forth.
However, most schools still wanted to receive a bit more personal information about candidates that looked OK in the primary app, but about whom they were unsure if they wanted to interview yet. The secondary application for a specific school usually asks for more short essays, often on the theme of "
why this school in particular?" It is a way to differentiate between those who are truly interested in attending there and those who just applied shotgun style and applied everywhere without actually investigating schools.
The $$$ is a big part of it, too. I guess having an admissions committee and processing all those applicants and doing interviews cost a lot. Not all schools have secondaries. Some don't ask for any additional detail, but just want your $75. Filling them out sucks, but it acts to whittle down the applicant pool since a lot of would-be applicants end up not bothering.