It's to signify that the places that houseless people reside, are in fact their homes, and each daily "sweep" they face is a grossly indecent, intolerable, violent violation of their right to exist. It signifies that this must be stopped, by any means necessary, and that the armed gangs of the state must be confronted in order to demonstrate real working class solidarity and house these people in all the vacant homes. It asserts that this is a solvable, specific problem in a person's life, not something abstract or essential to their "nature" or something that they "deserved". It is a way to rhetorically resist the daily, casual and explicit characterization of these people as sub-human.

But you're right, now these liberals will use it as a way to feel part of a movement that they violently suppress