I bought a separate GPS unit for my camera and use it extensively. Actually, I keep it pretty much permanently attached on the hot-shoe mount and have my pictures geo-tagged by default. I have a hard time imagining why you wouldn't want it.
I take a lot of outdoor pictures and am anal about keeping records. For years before I had this camera, I'd look thru maps and figure out where each picture was taken when I got back, and record the lat/lon coordinate. It is so much easier and nicer to have this done for me automatically.
Another side effect of the GPS is that the camera clock is always accurate. My camera sets its clock from the GPS whenever the GPS has a fix. This is often enough that the camera clock is effectively spot on all the time for my purposes.
With a GPS, every picture then really records where, when, and what it looked like. Ways I've used that, or reasons I've found it useful include:
- As I said, I'm anal about keeping records. I have my picture archiving software harvest the lat/lon from the raw file meta data automatically and include it in the human-readable information associated with each picture.
- Having pictures archived by location can be very useful for narrowing searches. If I'm looking for any pictures I have of historic Williams Barn, for example, I can limit the search to 200 meters from a lat/lon location I can easily get from Google Earth or various other maps and mapping software.
- It's been useful to figure out after getting home where something I saw really was. In some cases, I was able to figure out where I was and what path I actually took, even though I wasn't so sure at the time.
- I'm on the town Trails Committee. I usually take my camera with me when hiking on our local trails. If I see something that needs maintenance, like a fallen tree across the path, I take a picture of it and don't need to take any other notes. That one picture records what the problem is, exactly where it is, and when it was observed.
- I'm on the town Conservation Commission. I take my camera with me when we go on site walks. Not only is having the location useful later for sorting pictures by issue that we went to look at, but sometimes being able to go back a year later or whatever and check something from the same vantage point can be useful.
- In one case I saw something in a remote location that I alerted the authorities to when I got back. A picture was useful to show them what exactly I saw. Having accurate lat/lon coordinates not only helped them get to the location reliably, but I think they took me a lot more seriously than someone walking in saying "I saw xxxx out in the woods somewhere north of Jackalope Flats".
I had the reverse experience when my car got stuck behind a large tree that fell across the road deep in the national forest 20 miles southwest of Flagstaff AZ. This was before I had a digital camera with a GPS unit. When I got to the police station, I pointed out on the map where the fallen tree was. Their first reaction was "No, you couldn't have been there. Nobody goes there.". I had to argue with them for a while and explain landmarks I passed before they (at least pretended to) believe me. By the time I did get back to the car hours later, the fallen tree had been cleared, so in the end something worked. A single picture with lat/lon coordinates would have been really useful though.
- I used to enjoy uploading pictures to Google Earth. Having a lat/lon string I could just copy and paste made it easier and less error prone to place the pictures correctly. The reason I say "used to" is that Google is shutting down Panoramio, which was their mechanism for uploading pictures to Google Earth.
- I edit OpenStreetMap. A geotagged picture of something provides far more detail than any amount of note-taking (thanks Mark for reminding me of this in a comment). A good example is taking a picture of a gate at a trailhead or a stream crossing. OpenStreetMap has various tags for different types of gates and crossings. Looking at the picture while browsing thru the available tags has been helpful a few times.
Sharing Geotagged Photos
As Knob Scratcher pointed out in a comment, the location data in a picture file might give out information you don't want everyone to have. Actually, this is the case for all meta-data. Time and camera type are also things you might not want everyone to know.
So don't give people pictures with meta-data. The camera includes a lot of meta-data in the raw file. That has to be processed anyway. I have my software create JPG derivative files that contain no meta-data at all. Don't give out your raw files. Many people wouldn't know what to do with them, and bad post-processing will may you look like a bad photographer, not whoever did the post processing.
I do have my picture archiving and indexing software grab various pieces meta-data from the raw file and associate that data with the picture. The pictures and selected data is written to a HTML tree for easy viewing, navigating, and being able to display annotation with the picture. The software that exports selected pictures from the large collection can be told what type of data to export with the selected pictures. This way I can control what the rest of the world gets to know about each picture. So far this has worked well.