There is a certain "look" to black and white film that is difficult to simulate in a Photoshop color conversion; indeed you can spend hundreds of dollars on software that does noting but simulate film.
Also, black and white film already has all its tones and color responses balanced out; they're known quantities that respond reliably from roll to roll. With color conversions, you have to personally go through and tweak each image.
Another benefit of black and white over color is resolution and sharpness: black and white does not have to split its image over color dye layers, or any other tom foolery, so tends to have a sharper look, even if the emulsion is technically lower resolution.
Finally, tradition. Black and white is what everyone first learns in school; it's the medium that many important photographic artists have used (and continue to use), and it's a processes that's simple enough to do yourself at home.