On some early rangefinder lenses, a focus tab (also called focus lever) was designed as part of a lens focus aid. Essentially it is a protrusion of the focus ring that a finger rest on. Focus tabs serve two primary benefits.
One can learn to memorize the position of the focus tab and know the range of focus. This is great for street shooting where the photographer can quickly determine the focus point needed in order to focus on the subject properly.
On the Leica 50 Summicron for example. When the focus tab is positioned at 6 o’clock, the focus lies at approximately 6 feet. Quickly sliding it to 45 degrees to the right (counter-clockwise) will land the focus at infinity. Sliding it 45 degrees to the left (clockwise) will land the focus at around 3 feet.
Speed & Control
One can throw the focus much faster using a lens focus tab. The speed and control achieved by the dexterity of the finger is much more intuitive that holding the focus ring. When greater control and detailed focus is required (such as macro work), a lens focus tab offers the ability to adjust the focus with greater control.