The quick answer is: to make money.
The more involved one is that the new ammo adds a bit of velocity, a slightly flatter trajectory, and maybe better bullet performance to the lever guns. Not everyone or even most of the folks toting them these days are using irons and an extra 50 yards could be important at times, or maybe just seem that way. From the tests I've seen, the ammo pretty much delivers on the advertised performance, especially in longer barrels. Some of the older stuff often barely breaks 2000 fps. I gather that a fair number of .30/30 users prefer 150gr bullets in conventional loads for deer, feeling that the extra velocity helps with expansion and the terminal effect.
Hornady has a new LE load for the .25/35. According to the article on it by Brian Pearce, regular 117gr factory loads don't expand on deer and also don't clock as advertised. The new LE loads came close to the listed speed and expand well. Pearce took a pronghorn at about 200 yarda and the LE bullet (110gr) expanded and exited.
The LE bullets are also used in the .308 and .338 Marlin, which are a good bit zippier than the old levergun stuff, and close to rounds like the .300 Savage and .348. I know a man who has one of the .338s, and he thinks it's a hammer.
