A parfocal lens stays in focus as you change focal length. A varifocal lens does not, so focus shifts when you zoom.

The main benefit is convenience: with a parfocal zoom, you can focus at the long end (where focus is easier to judge) and then zoom out without needing to refocus. That matters most with manual-focus lenses and especially for video, where focus changes during a zoom are distracting and awkward to correct.

For still photography, the advantage is usually smaller because autofocus can quickly refocus after zooming. So for many photographers, parfocal behavior is nice to have rather than essential.

There are trade-offs. Lens design involves balancing size, weight, cost, speed, and optical quality. Making a zoom parfocal generally adds complexity, so lenses designed for this are often larger, heavier, and more expensive.

A related concept is focus breathing: some lenses change their apparent framing slightly when focus changes. That is also more noticeable in video than in stills.

So: pay extra for parfocal if you do a lot of video or manual-focus work, or if your shooting style benefits from zooming after focus is set. For general still photography with autofocus, it’s usually less important.