I raised this issue on the TeX on OS X mailing list a few years ago, and received the following reply from Herb Schulz, who created the basic autocompletion file for TeXShop:
Well, they will show up depending upon your input encoding. You are supposed to replace them by using the Next/Previous Mark commands to move and select them and replace them. When I go to the final • at the end of the environment I simply [h]it return and it gets removed and a newline is entered. I like to isolate environments. And you can remove them by editing the CommandCompletion.txt file in TeXShop if you wish.
Personally, I've never found them very helpful, but the idea is that you can move from one to the other and replace them easily.
You can read the entire thread here: Still trying to understand autocompletion.