1) You upload images to a stock site. You need keywords to help your customers find your images.
2) As input to a catalog app
3) Using the PM "Find" feature, assuming that is easier than just browsing the contact sheet. It would be helpful for me, for example, to find a particular bird species. I have enough trouble IDing confusing bird species in full size images. Impossible with thumbnails (except for very obvious species like Great Egrets).
Some species are so similar that they have to be confirmed by call or song or flight patterns at the time they are observed. In that case keywording the image is critical unless one has an extremely good memory!
In other cases I might shoot 50 images of a bird but only a few show the field marks that nail the ID. Keywording allows me to track the species of the other 40-some images.
That might be a niche subject but even with portraiture you may shoot people and need to record their names. Years from now you may want to go back and find them, but not recall their faces.
There are surely other usages I am just not thinking about at the moment. The value is in the eyes of the beholder and the beholder's needs.
Personally I think keywording becomes more important as the collection grows, and then passes the point where you can quickly aggregate then in a PM contact sheet.
4. Many people that do actively catalog their images in a dedicated app want the keywords embedded in the images, for backup if for no other reason.
You could "blow up" or corrupt the catalog, and not have a suitable backup. You might decide you need to switch catalog apps and you may not have the ability or the skills to somehow export the keywords you created in the catalog app or drive them back into the image files.
I can say from experience you only want to keyword a large set of images once!
I have taken to segregating my species by subfolder under my "original images" folder. It may not be the best way to do things but I find it easier than other methods I have tried, and I have absolute peace of mind that my species ID will only have to be done once. Just to say there are many ways to skin these various cats.
I think you should download the PM eval, install it, and get to know it better . Even if you decide to pursue a different solution you will learn something about keywording and how it might help you.