Because GPR can detect all targets, metallic and non-metallic and features like layers, rebars, and voids in the subsurface. EM locators will detect targets made from conductive materials and will need to energize other targets in order to trace them, which is not easy, nor flexible. Typically, GPR gives a deeper penetration, can provide B-Scans and A+B-scans of the subsurface and export this info into maps. If you have already an EM locator, adding a GPR will make your work easier, faster, and more accurate on site.