When my boiler was installed (it's a closed combustion device), the most convenient place to exhaust it and pick up the combustion air had two dryer vents and another exhaust from a furnace within close proximity (it's a townhouse)...IOW, I could not get enough separation from their outlets to my air intake; and, the outlet wasn't really in an ideal place, either. As a result, I ended up having them run both pipes to the other side of the house which was still within the limits listed in the instructions. So, there are numerous reasons why you may not want either the intake or exhaust in a certain place (door, window, overhang, etc., clearances must all be taken into consideration).
There are a lot of different termination configurations, including coaxial, that typically can be used. Regardless, they do need to be installed properly. The installer had the intake/exhaust on mine upside down and on a really cold winter day, the intake iced over, shutting things down. They fixed it, but it should not have happened in the first place.