I've never been personally invited or required to attend a business meal, but I can imagine that I'd have to put my best game forward. Meals can be grounds on which a prospective leader is scrutinised for their suitably or a business partnership is forged.
I've attended a couple of business dinners of this nature, albeit not concerning businesses that I was involved in - more as an observer as they weren't my business.
In my job as a HR Manager I've been to a number of dinners where it's been between colleagues and the evening dinner has been organised to eat whilst talking shop. I'm always mindful not to let my guard down and not to drink too much, as perfomance at these types of dinner is still judged. Even when people say this is 'afters hours', judgement is still being formed on your professionalism etc.