The problem with directly buying them food is that it has to be eaten within a fixed time window. Money can be used to buy food tomorrow, or it can even become food five years from now while being trivial to carry around. Getting money affords you the chance to take some time off from panhandling to attempt to better your situation because you know you have that backup food in your pocket. You're not going to get out of homelessness if leaving that street corner means you're probably going to be one day closer to dying of starvation.