It doesn’t know how much money you have. Many cards will allow you to pay small amounts even without checking the balance of your account. This means you may end up with a negative balance. Large payments are not allowed without an access to the bank. Varied amounts are considered small and large between cards/banks.
Some cards, e.g. Visa Electron, require the check every time. The benefit is that you can’t end up with negative balance which is why this kind of cards are usually recommended for teenagers and young adults. The downside is that you can’t use the card if the shop doesn’t have an internet connection.
Edit: If you’re paying something in an app by providing your card’s information, the app must have an internet connection and it is then able to check your balance before accepting the payment. The physical card doesn’t store your money. It’s only a way for shops to identify you as the owner of a bank account so that they can charge you.
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