Lead is very dense, so a bullet the same size delivers more force. Steel was an early alternative, and in addition to being less dense, it also tends not to deform into a mushroom shape during impact like lead and copper do. So a steel bullet can penetrate cleanly through a target leaving a narrower path and have a slower penetration and exit velocity, all combining to do less damage.
The density benefit is the same reason some high end rounds use tungsten and even depleted uranium. Heavier bullets of the same size fly straighter and hit harder.
Steel and copper also tend to cause more barrel wear than lead. Probably tungsten does too, but I don’t have experience with it, let alone depleted uranium, lol.
It’s still worth switching away from lead though.
Of course it isn’t an energy multiplier. The energy carried as the bullet leaves the barrel will be very very close to the same. The energy it carries will be lost more quickly to air resistance with a less dense round though. Especially since the same powder behind less mass, means more speed, means more air resistance. How much that matters depends on a lot of factors, but it can certainly be enough to matter.