Taking people out to lunch is a common professional-level behavior, especially for anyone involved in something in or adjacent to sales and service. Its called relationship building, I think is the most popular current term.
It is also generally something someone in such a professional position can expense. Free lunch for you, and also free lunch for them because they can expense the whole thing as a business lunch. And if it helps them develop a relationship in a way that is profitable for them, double-bonus. But if not - again, free lunch for them, too.
If you don't want to go, you are not obligated and can politely decline if you like. But if you are open to going it generally isn't something terribly weird in my experience, and you should get use to business lunches if it isn't something you've done before. Its just a chance to chat and, well, eat. It should go without saying you'd still need to treat is as a business function, dress appropriately, etc. - just because its a casual lunch doesn't mean it isn't a form of interview on its own.