It gives a semi-sheltered area outside the pilothouse while underway. A secondary benefit is that it provides a "safe area" or handhold when it is necessary to be on the foredeck in inclement weather.
A popular feature is a "Portuguese bridge", which consists of a walkway behind the foredeck, in front and to the sides of the pilothouse windows, separated from the foredeck by a (generally) waist-high bulwark. The purpose of the Portuguese bridge is to deflect green water from slamming against the forward windows of the pilothouse.