Well, imagine you're building something awesome/important to you. You get instructions for the measurements but who needs them?! Not you, of course! you eyeball it but make it just a inch too big xD (or too small). you could keep going, but it looks insanely weird. imagine this is a chair. if you sit in the chair but made one of the legs to big its going to feel uncomfortable and probably fall over.
I hope that helped somewhat lmao