Back in 19-aught-80 or so, when Shimano first announced the "cassette freehub," it promised to simplify a popular trend for customized freewheels. Suntour had their cog board. Regina had a similar layout. But ratcheted freewheel bodies required cogs with different internal diameters and attachment methods, splined or threaded, depending on position.
The cassette body moved the ratchet to the hub. This shifted the right-side bearing outward, to support the axle better, and made all the cogs except the last one splined with identical inside diameters. A rider could construct custom gears at home from a single size range of cogs. I thought it sounded great.
The drawbacks became apparent when trying to remove the last, threaded cog that held everything onto the freehub body, but the Hyperglide system with the separate lock ring cured that. But for many years now Shimano has not made the full range of cog sizes available.