Let's say you pay some people to design a building for you. You get insurance of course. A year later, the building falls down. You call your insurance company to collect. The first thing they ask is whether you took every reasonable precaution you could to prevent that from happening. Part of that would include having a licensed structural engineer design the building. If you let Joe Shmoe off the street design your building and it falls down, the insurance company is going to say you're basically SOL, because you didn't find a person you could verify was qualified.
It's really about transferring the liability from the building owner to the engineer. If there's a mistake in the design, the engineer gets sued. Guess what happens then? The engineer has professional liability insurance to pay that! How does the professional liability insurance company decide who to insure? Well, they insure people with licenses, of course. It's an elaborate scheme to transfer money from one insurance company to another.
Is it necessary for a florist? Probably not. I can't think of a situation where you'd need to sue a florist. Interior decorator might be more problematic than people realize. First, it depends on whether it's a decorator or a designer. A designer may move walls around, affecting egress paths, and can possibly have some life safety implications, so they should be able to take responsibility for that.