The main purpose of these washers is to help with FMQ. I wouldn't say I had a problem with FMQ before getting the washers, but I do believe my stitches are nicer with them.
For some quilters and some machines, I think they can make a world of difference in FMQ. For others, not exactly a "must". But hey, they can't hurt, they might help, they're not overly pricey, you get several in a pack and each one last a looooooong time -- so why not?