Why do people chew gum and are they really chewed gum, those thousands of round black blots on city sidewalks like NYC?

I’ve been meaning to ask this question for a long time and kept forgetting. Heh. But yeah, aside from wondering why people bother to chew gum, I’ve also been perplexed as to what those round black patches are dotting city sidewalks in NYC. I always thought they were spit out bubblegum but every time I look around I don’t see anybody chewing, much less someone actually spitting a gum out.