A swatch in Illustrator is stored in a library, generally of similar themed items so you can find say a selection of rose tinted swatches in one or a selection of red ones in another or perhaps gradient themed ones as well as pattern related. They can be gradients, patterns as well as solid colors. So how to open them ?
You can open the AI files (they are all stored in AI format) via the file open command. The only trouble with this is that the items will only then be limited to the current document. You can copy the items from this file and then add them to another document and then drag those paths into the presets panel. A bit of a workaround.
If you want to open them as swatches then you have to use them via the panel and use the right side menu to load the items / AI files. You can also open them via the window menu and the swatches library option. They will appear in the list if they have been added to the presets 'swatches' folder but if not then you will have to use the other library option. Another way to open them is to have at least one library (they are read only) displayed and then use the bottom panel to either open the set or use the arrows - back and forward to load the named set. They will only appear if they have been added the AI file to the presets folder otherwise you will have to use the other library option. Any of the libraries opened are read only and you can only select items from them, you cannot delete them via the application.
Personally I find this the most useful option as I generally don't have the files stored in the presets folder but I have them all in a central location as well as on other resources. If I change versions of AI then it means I don't have to have multiple copies, they can all be located in one folder or location.
How to open the swatches file as a normal file and use the resources in another. Tutorial showing how to open and use the swatches file [Youtube]