The reason is not distrust and it is worth explaining, because a deposit can feel like being asked to prove something. Project work has a period at the beginning where a lot of effort produces nothing visible: understanding the business, working out positioning, planning structure. If that period is unpaid, the incentive is to rush through it to reach something demonstrable, which is precisely backwards. A deposit means the thinking gets the time it needs.
It also protects you, which is less obvious. A provider with no money committed has nothing anchoring the schedule, and your project competes with whatever arrived more recently. A deposit creates an obligation in both directions. What you should expect in return is a stated start date, an agreed scope, and a schedule you can hold somebody to.
What matters more than the deposit amount is what surrounds it. Written scope before payment, so both parties know what is included. A payment schedule tied to stages rather than dates, so money moves when work does. And clarity about what happens if the engagement stops partway, including what you keep. You keep everything produced to that point, and every account is already in your name, so stopping is a conversation rather than a recovery operation.
If you are comparing providers, be cautious of anybody asking for the full amount before starting, and equally cautious of anybody asking for nothing. The first leaves you with nothing to hold back if the work disappoints. The second usually means the work has not been scoped carefully enough for anybody to know what it costs.
The amount itself is worth thinking about in terms of what it covers rather than as a percentage convention. A deposit that covers the work you would do before anything is deliverable, plus any materials you have to buy upfront, means an abandoned project does not leave you out of pocket. That is usually somewhere between a quarter and a half depending on how front loaded the work is, and stating the reasoning rather than the percentage tends to make the conversation straightforward.
If a deposit is genuinely a problem for a client, that is information rather than an obstacle. Sometimes it means cash is tight and a payment schedule with smaller stages solves it, which is a reasonable accommodation. Sometimes it means they have been let down before and want to see something first, which a smaller initial phase addresses. And occasionally it means they are not in a position to commit at all, which is better to establish before the work than after.