The advantage of fixed lenses is that they are (1) less costly, and (2) smaller. Therefore, you will see that IP cameras with fixed lenses generally will be cheaper and more compact than cameras with varifocal lenses.
However, since a fixed lens cannot be adjusted, whatever the camera captures is what you will be given. Though, note you often can manipulate the image and crop or cut out that part of the image you’re not interested in.
Lenses, particularly good ones and varifocal types, are expensive and form a significant part of the cost of a camera. This explains the significant price differential between CCTV cameras with a fixed lens versus cameras with a varifocal lens.
Varifocal lenses have parts that can move in and out, and therefore require more space within the camera enclosure, hence varifocal cameras are almost always larger than fixed cameras.
The big benefit of varifocal lenses is that you can precisely set your camera view and capture the area you are interested in (much) better detail. This is particularly important for monitoring people, vehicles or activity at a greater distance, or when the application requires clear detail, which is often the case when using video analytics, such as face detection or ANPR installations.