Application of Spar varnish on interior window sills
Spar varnish is optimized for outdoor use. It isn't as tough as indoor varnishes, but it is more flexible so it survives weatherng better, and it generally contains UV blocking additives which (partly) protect it  from chemical breakdown due to sunlight.

If that trade-off is acceptable, there is no reason you can't use it indoors. It may not be the best choice, but depending on your needs it may be more than good enough.

When I redid the trim around one set of windows, I explicitly chose to use spar varnish as the top coats because part of the trim is effectively outdoors and I wanted that weather protection. Then, since I happened to have it on hand, I've used the leftovers on some other projects where I just wanted the look of an oil-based varnish and wasn't especially worried about friction wear.

Tools for tasks... But if the tool works well for the task you need to do, go for it, even if that isn't what someone else uses that tool for.

Note that spar varnish is generally oil based, and will add color that water-based poly does not.