Hey axxe, sorry I haven't chimed yet. I must have missed your post.
Hyphenated has explained the 45-70 pretty well but I'll share my own viewpoint. I initially bought one due to lust for the long barrel of the 1895CB when I couldn't justify the expense of a Sharps replica. After having spent time shooting it, mostly at the range but some successful hunting too, I have found it is the one cartridge I can't do without. The other calibers in the cabinet are useful and fun but disposable to some degree.
The 45-70 is so versatile and successful as to make me a disbeliever that our federal government was ever responsible for having commissioned it into existence. Even at it's inception it was produced in at least two variants of powder charge and projectile weight. Also, a cartridge that has been in use, continuous use, for over 130 years will have experienced extensive testing and evolution by shooters of all kinds. And it still has shown there is more left to discover with it by ingenious tinkerers. My wife even owns and shoots one. She too has discarded the factory ammunition for one of my home made rounds that suits her and the rifle better. The same rifle she uses with mild recoiling cartridges to harvest deer in the woods could be loaded with grizzly bear level armageddon rounds that will take out elephants and cape buffalo.
There are a lot of wonderful cartridges in the world that do stupendous things with a lot of variety. Without prejudice I can honestly say there is no other round that can perform with such versatility, begs to be customized by the reloader, and has so much panache of history accompanying it. It offers more for more people than anything else. And if you want to use yours for one, single, focused purpose it will please you that way too.