I don't fully get the use case either, but it's in a different category than Postgres or Microsoft SQL because it runs in the browser and can be made part of your web app.
It's targeting use cases where you want to push analytical computation away from servers into the client (browser).
Lets me sketch 2 examples:
A) You have your data sitting in S3 and the user-specific data is in a browser-manageable area.
(E.g. this paper from Tableau research actually states dataset sizes that should fall into that category: https://research.tableau.com/sites/default/files/get_real.pd...)
In that scenario, you could eliminate your central server if your clients are smart enough.
B) You are talking about larger dataset sizes (GB) and want to explore them ad-hoc in your browser.
Uploading them is unrealistic and installing additional software is no longer ad-hoc enough.