The buyer/owner of a call can "call away" the underlying FROM the seller/writer.
The buyer/owner of a put can "put" the underlying TO the seller/writer.
Somebody who's short needs to buy the underlying to close out, so selling the put is what he wants.
1) If market goes down, below the strike, then he's in the black; even if he gets assigned the underlying, that's exactly what he wants, and now he's got a lower basis, winning trade.
2) If market moves down a bit, ranges, or goes up slower than the received premium, then that's good, and he can do it again (if using the shorter expirations).
3) If market moves up too fast, then he's lost more. That's what Tom cautioned:
"What you need is a reasonable expectation that you can raise that strike/cash faster than the market can raise it's mark, week after week after week (without fail). So! You need 12pts. What has the market average been?"