There are two types of homeless: (a) there are the temporarily homeless, who have lost their homes due to unaffordability (lost job, unforseen expenses, etc), may be fleeing domestic abuse (#1 reason for women with children), or some other temporary condition that prevents them from acquiring or maintaining housing; and (b) there are the permanently homeless, who have lost their jobs due to mental disease and/or drug use. (Note that the use of mind-altering substances causes physiological changes to the brain, so drug use frequently leads to mental disease.) Most of the (b) group chooses (to the extent they are able to exercise choice) to remain unhoused, because shelters and housing forbid narcotics, alcohol, and other contraband.

Right now, we operate under the ridiculous presumption that members of the (b) group are capable of acting rationally and in their own best interests. This is objectively false, but since this is the presumption we are forced to pretend is true, we can't force them to into treatment, however bad they may need it, unless they are at risk of inflicting imminent physical harm to themselves or others (but in this context, physical harm doesn't mean any harm typically arising from drug use...)

The proposal lets us force the members of the (b) group into treatment/rehab, however much they may refuse. And this is important, because people who are mentally ill will refuse treatment, until they get treated, at which point they recognize why they need to be treated, and will generally choose* to remain under treatment and to accept housing.