I think it matters because the whole application process is very subjective. If you worked at a hospital for three years, you will be seen as someone who got a lot of clinical experience, it can be a very important aspect in your personal statement, you can get great recommendations from a physician (as opposed to someone who just shadowed for a few times), it will be hard for anyone to question your desire as a physician if you have worked around that field...etc. I dont think you can have these advantages if you just worked as a waiter. However, it is not a negative to work at other fields to earn a wage, but it just wont give advantages that certain healthcare fields can give you. Even a lab assistant is helpful because it gives you research experience, more knowledge about some field, get to know the professor better...etc. If you have a choice without other limiting circumstances, go for the job that helps your application out.