It's not a VOA, but Visa exempt. Your airline might, or might not, require you to have proof of your leaving Thailand within the 60 days as I've heard. I've personally never been asked for that proof but haven't tried to enter Thailand for almost 2 years, and they may be a bit more strict about it now.
if you get in a bad accident whilst in Thailand, you probably won't be able to work, right? So they need for you to be able to pay your hospital bills before you leave the hospital. Need proof of being able to care for yourself.