since deposit bonus is not withdrawable, then what's the point?
Although the free deposit bonus is Non withdrawable, it still adds to your margin level, keeping your trades open for longer when the market is moving against you.
To add to what @dominykas said, imagine your broker has a 20% stop out level and your margin level has fallen to 25%, you will be at the verge of being stopped out of the market. But if you have a deposit bonus of, say 50%, it will boost your margin level to 75% thus buying you more time in the market for the tides to change.
Deposit Bonus vs No-Deposit Bonus
The broker puts the "deposit bonus" into your trading account so it adds to your free margin. However, for the one they call "no-deposit bonus" be careful because that one is not put into your account, so it doesn't add to your free margin.
The broker will usually create a separate account for the no-deposit bonus, and you trade from that account.
It adds to your account equity, so that you can set a wider stop loss on trades than you would have without a deposit bonus.