I think the big difference is that a mid drive motor powers the bike through the gears on the back wheel, so they can leverage that gearing advantage just as a rider's legs would do. It's a big advantage for hill climbing, but also in other situations. It's also a lot easier on the motor.
That said, mid-drive motors are more complicated to install. You might check out some YouTube videos to get an idea of the process. There are a million of 'em.