From my understanding (SOMEBODY CORRECT ME IF IM WRONG) Eevee is designed for realtime rendering in mind - it uses tricks similar to videogames to get high quality renders without raycasting everything.
Yes, Eevee is a fast pbr renderer and can give very similar results to cycles with no effort, except there's some setup needed to do indirect lighting and reflections... and some tweaking needed for good lamp shadows in some cases, but the payoff is huge. I have a high resolution render that takes 12 minutes in cycles, 30 seconds in eevee with no grain... some features are not there line a shader bevel, but otherwise very similar, and there's workarounds for that.
I used to switch to internal when designing on really low-end hardware (Atom tablets)