The "biggy" effect of direct injection, is the "diesel" manner, that the ICE (internal combustion engine) then changes to, versus port-injection, or the old carburetor air-fuel behavior in the cylinder.
In a port-injector (same as like a carburetor), the cylinder compresses both the air and the tiny droplets of fuel, during the compression stroke, the fuel droplets typically boil to become fuel in full vapor state.
Fuel, like water, has a dramatic (for this blog lets guesstimate it at about a 1000/1) ratio of volume/pressure increase when changed from liquid to gas. That added fuel volume/pressure pushes back on the piston, during the later half of the compression stroke. That's a bad thing. It reduces "thermal efficiency", in a big way. That means more waste heat, and less mechanical engery per gallon of fuel.
So.. along comes Mr Diesel, and direct injection. Liquid fuel is directly injected into the cylinder, near the end of the compression cycle, but it has to be under extreme pressure to do this. And it requires an extreme fuel pump, and extreme fuel plumbing. The direct injected fuel, heats so fast, upon injection, at near the end of the compression cycle, it often does not need a electric spark, to set it off in spontaneous combustion.
And thus there is also LESS back pressure on the piston (this is the desired effect), during the majority of the compression cycle, and thus improved thermal efficiency.