Someone emails about your space for a 120-person party. By the time you've traded six messages about date, headcount, and price, they've put a deposit down at the venue that answered faster.
A deposit is the line between a real booking and a maybe. Collecting one means another tool, another invoice, another thing to chase.
Every event needs details you don't have yet. Guest count, setup time, whether they're bringing outside catering. That information arrives in scattered replies, when it arrives at all.