What’s the use case? Printing out filled-in forms? But otherwise, who would want the PDF in electronic format? It doesn’t seem like a practical way for users to submit data.
Are there utilities that extract PDF field data and submit it to a database? I'd be grateful to see examples.
What about field validation? The PDF may have some minor validation, but that's no substitute for the validation done in a DBMS.
If you want users to be able to save a nice looking form, you'd still want the data entered online directly into a DBMS. I'd offer a "download PDF of your input" as an option, for example.
That said, plenty of users of PDFs have a very paper-based/manual workflow still, and not the motivation and expertise to run and update an online form thing. Or they need to have the ability to handle odd inputs anyways, because paper forms have even worse input validation.
And from a browser/user perspective, the feature here is useful because people expect me to handle PDFs and do not provide nice web forms. They might have terrible reasons for doing so, but I still need to live with that.
A lot of people don't care, because they come from forms in cartaceous - where they have to manually retype everything anyway. For many, their "DBMS" will be an Excel sheet with a dozen rows. The more advanced types likely have some Adobe software that does all the magic.
Fillable PDF forms are really seen as a courtesy to users more than anything particularly useful to the emitter.