The beam width bounds the memory required to perform the search, at the expense of risking completeness (possibility that it will not terminate) and optimality (possibility that it will not find the best solution).
The beam width bounds the memory required to perform the search. Since a goal state could potentially be pruned, beam search sacrifices completeness (the guarantee that an algorithm will terminate with a solution, if one exists) and optimality (the guarantee that it will find the best solution).