Modeling before machining
Every physical part of a robot, a bracket, a gear housing, a leg segment, exists first as a digital model. CAD, computer-aided design, is the software category that lets an engineer build that model with exact dimensions rather than a rough sketch. A part modeled in CAD isn't just a picture of a shape, it's a precise mathematical description of every surface, edge, and hole, which means the software can answer real engineering questions before anything gets cut or printed: does this bolt hole actually align with the one on the part it's supposed to attach to, does this bracket clear the motor next to it, does the wall thickness leave enough material to survive the loads it'll see.
This matters because physical mistakes are expensive and slow. Machining a metal bracket, 3D printing a housing, or molding a plastic part all cost time and material, and if the part doesn't fit once it comes off the machine, that cost is wasted and the schedule slips. Catching a dimension error, a collision between two parts, or a hole that's a millimeter off in CAD costs nothing more than a few minutes of screen time. That gap in cost between fixing a mistake on screen versus fixing it after fabrication is the entire reason CAD exists as a discipline.