This was in response to the French adoption of a pointed and boat-tail bullet, which offered better ballistic performance.
The combination of tubular magazine and centerfire ammunition has been referred to as "too excitable", especially when used with pointed bullets.
Heavy-for-caliber pointed (spitzer) bullets with a boat tail design have high BCs.
Bertie-Clay is most famous for being the inventor of the dum dum, a soft pointed bullet.
There also exist lever-action rifles that feed from a box magazine, which allows them to use pointed bullets.
The role of that circular groove is to receive the tip of the pointed bullet that follows when loaded inside the Lebel's tube magazine.
It features a bottlenecked case and a pointed, steel-core, brass-jacketed bullet.
This allows the use of aerodynamically-efficient pointed bullets (as opposed to the spherical balls used in muskets) and thus improves range and accuracy.
Generally these loads would use more aerodynamic pointed bullets, the same type used by .22 caliber centerfire cartridges.
This means that it slows down faster than pointed bullets.