Why do I have to pay a deposit?

This is a sign of good faith. Properties hold rooms for you, which they could sell to someone else, so if you don't turn up, they lose revenue. By paying a deposit of 50% (industry standard) you show the property that you are intending to arrive and they will accordingly turn others away.

Can I pay the deposit at month end?

That depends on the property. Many have real time booking calendars, allowing people to book instantly online. If that's the case, bookings are first-come-first-book, and no rooms will be held provisionally unless specifically arranged. Send a request and ask for a specific payment date and they'll get back to you.