Bypasses are good because they reduces the amount of traffic in the centre where the people live and work. It makes it much safer for them to cross the roads, and reduces pollution. It is better for the vehicles passing the town because their journey is made quicker.
People are not always happy when a bypass is built. The main objections people sometimes have is that a new road uses up a lot of land. Also, some people who have businesses in the town (especially filling stations and restaurants) may think that they will not get so many customers.
If there are no strong rules that control the use of land, buildings are built along a bypass, making it an ordinary town road so that it might become just as busy as a town street again.
A road that goes all the way around the town in a large circle is called a ring road. The M25 is a ring road for London.