You need to report it because the local police team needs to know where to send more regular patrols. Call the non emergency number and have a chat with the local team. They are very helpful. Even if you can't describe them, the team needs to know of patterns and where these things happen.
I live one street away from all these reported muggings. I walk home late at night and after 7 years of lving here it is only in the last 2/3 months that I have started to feel intimidated by groups of teenagers (not anticipating something to happen but actually having something happening).