Sub-lieutenant emerged as ships needed more junior officers

When and why did the Royal Navy introduce the sub-lieutenant rank?

It emerged when growing ships and officer establishments needed a grade below lieutenant instead of relying only on numbered lieutenants.

The distinct sub-lieutenant rank appeared as the navy moved away from relying on first, second, third, and later numbered lieutenants. Ships and staffs required more officers than the older progression handled neatly. A grade below lieutenant gave the expanding officer corps another practical step. The change reflects organizational growth rather than a simple renaming of an older rank.