Write Keyword Tag and Properties Info to Photos ???

You are right that a lot of properties do come from, and are stored in the file by your camera.
Your personal organization input: keywords, captions, ratings, albums, creations are NOT written by default to the files themselves. They are stored in the catalogs. Working with catalogs (adding, modifying, searching) is so much quicker than rewritting the files themselves!
That's why writing the metadata to the files is only optional.
There are special keywording situations in which I have to add a temporary tag to nearly all the files in my library (>450 GB). That will take hours to write to files, seconds to add and remove in a catalog.
Since that takes time, you can choose the best moment to 'write metadata to files'; it's a good idea to do this just after importing and editing a batch, just before a backup or a 'Sync'. With big libraries, you need backups which require hours. I don't do a backup after each editing session, only periodically, at night. After each significant session I use the Windows 'Synctoy' to sync my library to another external drive. A matter of minutes only. That way, after a few hours of editing, I select the imported batch (very useful sort order in the organizer), write metadata to files (a matter of seconds) then launch the 'Syncing' and take a coffee...

Tips:
- you can either change the IPTC part of the metadata on your files from the editor or from the organizer;
- writing metadata to files only adds keywords, does not replace. The trick is to 'wipe' those IPTC data from the Properties panel of the organizer before writing the changed ones;
- The 'Save for Web option' deletes most of the metadata to get smaller file sizes.

I have not tested the captions methodically in various versions. The process may have changed. Today, my advice is to consider them like keywords and to use the 'save metadata to files' command to be sure they are recorded in the metadata section of the file.