If the battery is depleted somewhat after acceleration the engine provides power to the electric drive motors and to top up the batteries very efficiently as the engine will be running at its most efficient while doing so, unlike a directly mechanically coupled engine to wheels setup via an inefficient and power sapping transmission. Constantly varying the speed of an engine from near zero revs to near maximum is highly inefficient.
A largish car doing 70mph only uses about 20-30% of the power of the engine. Most of the time a car's engines is way oversized and carrying all that surplus weight. The surplus power is for acceleration. Electric drive and kinetic brake recovery using supercapacitors and a decent battery set can mean engines are much smaller. Having range extenders means the engine can be built light, small and optimised to run at its sweet revving spot.