You can use Numbers for a wide variety of purposes, but not with the File Generator.
The File generator is only useful for one single file of text which makes the point of publishing anything from Motion that uses it practically useless for Final Cut. You cannot publish the File source or Browse menu/chooser. Changing the display contents always requires going into the original file and making changes there.
If necessary, a File Generator can make creating a Subtitles track easier.
Simon Ubsdell uses it for "Word Cloud" effects. (https://www.youtube.com/watch?v=NVO-ezWhPgc)
In general, there are no objects in Motion that "recognize" numbers as numbers — there's no way to "parse" numbers out of any kind of text (which is unfortunate in some cases, like reading metadata from FCP). There are no text objects in Motion that will convert a Numbers value to text either — no way to add/insert a numeric value into a "string" of text characters. To pull off that illusion it is necessary to use one or more text objects aligned to a Numbers object (or vice versa).
For the most part, if you're just learning Motion, you can put off learning about the File generator... I think I've used it, like, 3 times in 17 years.
Sorry if this disappoints.