The reason people care is human nature. People want everyone else to like what they like. For many of us, we've loved comics for decades. Before they were viable as these movie properties that we always wished and wanted to see.
Now it's like this geeky stuff we loved since we were kids is widely accepted in the mainstream. It's validating
Rotten Tomatoes is an industry standard now. It's become a big part of marketing films. Studios even promote tomato ratings on home video covers now
For smaller indie films, it helps get their films more attention if they aren't getting lots of theatrical play.