But what comes off the build platform is generally not as strong as cast or injection molded parts. (There are some exceptions like the rocket engine printed by NASA but that's not what we're talking about here.)
For mere mortals on a budget... it's about practicality and augmenting your prints with strong, plentiful, and inexpensive parts whenever possible.
Need to make a robot rock and roll? A 3D printed ball bearing won't be round enough, so let's use one of these steel ball-bearing casters:
Grab your calipers and let's get to it.