Internal focusing mainly means the lens changes focus by moving internal elements rather than extending the whole optical unit.

Practical advantages:

• Faster autofocus: less mass often has to move, so AF can be quicker.
• No barrel extension: the lens keeps the same size while focusing, which is convenient in use.
• Less air pumping: because the barrel doesn’t move in and out as much, there’s less “bellows effect,” so less dust or humid air may be drawn through the lens/camera.
• Better control of optical correction at different focus distances: moving internal groups can make it easier to manage aberrations over a wider focus range.
• Framing stability: IF designs can reduce changes in angle of view/magnification while focusing, so composition shifts less.
• Often helpful for filters because the front of the lens is less likely to move or rotate, though that benefit is not exclusive to IF.

Trade-offs:

• It’s a design choice, not a free upgrade. An IF lens may be more complex, and non-IF lenses can sometimes be more compact when stored.
• The optical/mechanical layout is different, so any benefits depend on the specific lens design.

So the “big deal” is mostly handling, autofocus, cleaner operation, and more consistent optical behavior across focus distances.