Metadata: UserComment, ImageDescription, Description; what's their purpose?

$ exiftool test.jpeg | grep 'This is'
Image Description               : This is the 'ImageDescription'.
User Comment                    : This is the 'UserComment'.
Description                     : This is the 'Description'.

$ exif test.jpeg | grep 'This is' 
Image Description   |This is the 'ImageDescription'.
User Comment        |This is the 'UserComment'.

$ exiv2 test.jpeg | grep 'This is' 
Exif comment    : This is the 'UserComment'.

Three different commands show three different subsets of these tags.

Just to add more confusion, when the image is uploaded to Google Photos only the "Description" is shown.

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What is the intended purpose of these (and perhaps other) similar descriptions, and in practice how do people typically use them?

Note that I’m not asking about the technical details of differences in how they are stored as meta data; that’s easy to look up.
I want to know why one would decide to use one rather than the others, or to be safe should one always fill in all of them with the same information?