Are beets related to turnips

I think it is mostly for the taste. It certainly isn't necessary - I roast beets without any vinegar all the time, and they turn out great. I just roast them in their skins, without any oil or anything added. It takes a long time, over an hour depending on how big they are. When they are completely tender, let 'em cool, and the jacket slips right off. Roasting intensifies the flavor by caramelizing some of the sugars and removing moisture. You can always season with vinegar after roasting, depending on what you plan to do with them.

Roasting caramelizes the natural sugars in vegetables and, eventually, burns them. Roasted anything that turns out bitter is almost always the symptom of overcooking. Sometimes it also means that your ingredients were spoiled or "off" in the first place.

As for saving them - there's not much you can do in either case, other than trying to mask it with some other flavours. Beetroot pairs well with cranberry and other tart fruits, which would probably mask the bitter taste; cream, sweetened yogurt or sour cream would also be a good bet. Otherwise, I can't really think of anything else to do with them; overcooked food usually ends up in the trash for me.