Only do this if you are flying an approach to an MDA and using VNAV coupled to your autopilot to controll your descent rate to the MDA. adding 50 ft to the MDA will give you a DDA (derived decision altitude). If you are using V/S to descend just use the published MDA.
Since the 737 has VNAV and IAN (if you select it) you can for example, fly a VOR approach with the autopilot tracking the VOR for lateral guidance and VNAV controlling you vertical guidance. In this setup it would be appropriate to use a DDA (MDA+50).