C'mon guys, why are we downvoting these 2 without even giving them an answer? I would take a shot at this but I haven't followed e10s development that closely, and this is beyond my area of expertise.

I would just say that the biggest difference that I can see when I enable e10s is that the browser feels smoother - little things like the tab spinner no longer lagging, no animation lag upon opening more tabs when a tab is currently loading etc. Other advantages that I understand are that one tab crashing no longer takes the entire browser down and that e10s would pave the way to sandboxing. Multiprocess is also something that all (?) major competitors of Firefox already have.

Bear in mind that the responsiveness improvements may not be that visible on high end systems, but on my end - a budget laptop from 2009, further loaded with 35 add-ons while multitasking with other heavy apps - I can tell that there is a very noticeable difference when I enable e10s. And this is when most of my add-ons are shimmed (aka not fully e10s compatible.

Hopefully, you would some get better responses. Until then, you can read this: The “Why” of Electrolysis.