Skip crew.. they are a bunch of little leeches. Worse than pirates.
Think of Engineering like this : You bought a REALLY nice sports car, like a Porsche GT3. Very nice car, and you made sure to tick all the boxes for top of the line factory stuff. But you want more.. always MORE. And you heard about this place called RUF, that will do seemingly impossible things to your car, for a price. The Difference between a normal Porsche GT3 and a RUF modified GT3 ? Night and day. But you want more.. how about carbon fiber panels to save weight ? Or some really sticky tires on ultra light rims ? That's Engineers and Engineering. The best you can buy, and just a touch more...
Engineering is the last step for your ship. You can get a LOT more out of your FSD once it's been to Felicity Farseer. Thrusters need a bit more thrust ? Prof Palin can fix you right up. Power Supply not powering enough ? There's an engineer for that too. In fact, there's an engineer for almost any system ( you cant engineer guardian stuff ). But, you need an introduction from a faction or other engineer, or a rare good, or some other requirement before they'll see you. And you'll also need all the MATs for the modifications. So, MATs traders.. It's a long road.
Before you start be warned ! This is not a journey for the faint of heart. It costs in time and MATs... in fetch missions.. in buttering up others for the introduction.. and FINALLY, in terms of your soul, as you dream endlessly of that last tiny percentage you can squeeze out of your chosen ship. Like the CMDRs that compete in Anacondas that can make 84+ LY jumps. Or Cutters with over 15,000mj shields. Or 700ms+ boost speed Eagles interceptors. Or 6k range beam lasers that run cool and have no damage drop off. Or Insta-gank FDLs.. All engineered.