While it would be kind of handy to have someone around who can answer these questions, my guess is they would have reference materials at their desk to look up the answers. Nowadays of course I would just google them and find the answers.An engineer in your field (structural) will be able to tell you if a 1/2" bolt made from stainless steel with 8% chromium will hold up a 500 pound cabinet without collapsing when a 300 pound person sits on the top. An engineer (materials) will be able to tell you the tensile strength of a blind rabbit joint made from Ipe using Titebond III so you can hang a 50 pound planter from it without dropping it on someone's head. An engineer (mechanical) will be able to design a new joint style to solve a particularly vexing problem you've had with a standard joint in a non-standard application. A chemical engineer will tell you what to coat the wood with to keep out pests and remain handprint free over 20 years.
My wife's uncle is a manufacturing engineer, and he really has problems justifying his salary. I don't think he's ever worked a job for more than 5 years.