Effect of disabling dedicated graphics card in laptop

CAD will definitely be affected, as would any graphic design type software. To what extent I can't say, it depends entirely on the power of your onboard GPU and the complexity of your CAD project. For example, if you're modeling something simple the onboard GPU may handle it just fine, but as your design grows in complexity the program will become sluggish. If you're running Windows 10, which has a graphics heavy UI, it may become sluggish much faster than with an older OS. I don't run Windows 10, so can't say first hand. Movies (whether they be HD YouTube or BlueRay) also run off the GPU. Technically speaking, everything you physically see on a computer runs off the graphics card these days.

Bottom line, everything will initially work off the onboard graphics, but as you (the user) push those GPU intensive applications (CAD, Gaming, movies, etc.) the GPU will reach its limits. When that happens, you'll know you need to reactivate the dedicated GPU.