I echo what "hasarad" pointed out and would like to add the following from my experience.
The electric motor assists in the initial propulsion of the car from a dead stop. If the HVB has sufficient charge the EV light will display for the initial start and then you will hear/feel the internal combustion engine (ICE) seamlessly take over. All that will happen whether you are in NORMAL or ECO.
The most fuel used is in getting the 3,000+ pound Insight moving from a dead start. The electric motor essentially alleviates ICE from the heavy lifting and it's with the 197 lbs/torque that gives the car that added oomph to get it out of the gate before ICE takes over. Based on the drivers input and whether in Normal or Eco mode, it's the cars electronics/computers that determine when to best switch between EV and ICE for maximum fuel efficiency. However, the one thing the cars electronics can't do is predict - it can only react to what it faces.
For me I select EV mode when I want to force the car to run on pure electricity and do so as much as it can. I live in San Francisco and as you probably know we have a lot of hills out here. It's true ICE may wane going up those hills, but what goes up must come down and on the decent EV will lay silently making GREAT use of REGEN. Our year round temperatures averages between the low 50's to the mid 60's, so it's good for the body, the soul and for the BATTERY. San Francisco is 7 square miles and since a fair amount of our driving is within the city limits I've experimented how to best use EV mode with the routes we usually take. On a few routes using EV mode vs Normal or Eco, I've gotten as high as 113.4 mpg vs 48 mpg for a current drive. To achieve the high numbers I stay in the BLUE areas, keep my speed below 25 mph, use the Regen paddles for every stop and take advantage of coasting down the hills alternating between both +/- regen paddles for quicker battery charging. Also when I do use BRAKE HOLD, just before I'm ready to go I depress the brake hold button to release the brakes then press the accelerator. This way the electric motor doesn't have to work as hard to get the car going from a dead stop. That may not seem like a lot, but believe me it helps - it's plain physics.
I previously mentioned that the cars electronics/computers can't predict what roads are coming, it only reacts to the current situation. In my case since I know the roads and what is coming, I therefore know where and when I can use EV to its maximum, where and when I can get maximum charging and where and when I need ICE. Like J. Bond said, "it's a video game" and I'm having loads of fun playing! So having said all that, there is a benefit for EV Mode beyond just silently driving around the neighborhood quietly. A mile is nothing to sneeze at, I can get two!