A LLC is a smart way to go (though not the only way to go) if you need to protect your personal assets. Say, for example, you put dressing on someone's running board and the customer's foot slips off, they get hit by a bus, and they die. Or, say you use something to clean their interior and the customer has an allergic reaction. Or, say you're polishing an exotic sports car and you burn through the custom million-dollar paint job. Or...
Yes, some of those examples are pretty out there, but what happens if one or any of them happens and your insurance policy doesn't cover it or doesn't cover it fully? They sue your business. What happens if your business is only worth X dollars, but you're ordered to pay XXXX dollars? They come after your personal assets. A LLC will protect you from this. When a particular business model is vulnerable to a high level of liability, such as a detailing business, a LLC is a good way to go if you do or will have personal assets to protect in case something unspeakable happens - which it can. People are litigious and they'll sue for the just about everything. Remember the McDonald's hot coffee lawsuit?
Note, however, that a LLC does not protect you from financial liability. If you default on your business loans or credit cards, they'll come after you personally. If you business goes bankrupt, so do you. No business structure will truly insulate you from financial liability. Trust me, I know from personal experience, and I doubt I will ever want to own another business again.
Anyway, that's just my opinion on the matter, but don't quote me on it. I'm not a lawyer, nor do I play one on TV, nor do I do detailing for one. Do your due diligence and don't seek the advice of a lawyer, seek the advice of two or three lawyers.