Except when you can't.

Landlords very often just won't take you on at all if you don't show evidence of preëxisting stability. No proof of stable income? No housing—you can even offer to pay up-front or whatever, and they just won't bother dealing with you as a special case, because why would they deal with the special case when there's a dozen other safe, predictable, normal people in line behind you?

Got an income, but can't prove it to be consistently enough? Congratulations, no housing for you. And thus begins the downward spiral.