When you want a nice looking off-the-shelf feedback modal with a simple API without depending on Bootstrap JS and the entirety of jQuery.

In that case, why would I want a library that is specifically designed to work with Bootstrap?

I'm pretty sure this library simply forks SweetAlert to use Bootstrap's markup and Less variables. So it comes out of the box plugging into Bootstrap CSS and your overrides (like, I imagine, a Bootstrap theme from bootswatch.com).

I have a Bootstrap CSS + React app that uses React for everything I'd need Bootstrap JS for. The cost of Bootstrap JS is 9.9kb + 32.5kb (jQuery) = 42.4kb. Meanwhile, SweetAlerts JS weighs 3kb gzipped and comes with a simple API for exactly what I want to do.

OK, that makes sense. Not my scenario but I can see the purpose now.