I think that you are asking about the radar
beam width instead of bandwidth. The radar pulse spreads wider as it travels greater distance. The advantage to narrow beam width is that you can get better resolution.
If you are seeking a harbor entrance and the pulse is wider than the width of the opening the radar will paint
it as a solid shoreline. If it is narrow you will see the opening and perhaps a return painting the shoreline inside the bay.
The same thing applies to being able to pick out multiple boats in a group vs. them being painted as one return.
More details are here: http://www.technicalmarine.com/pages/marine-radar