Myself included - I'm not ashamed to say that my first conlang was an auxlang. I handled vocab by importing it from "the big five" (Classical Latin, Attic Greek, Classical Arabic, Sanskrit, Middle Chinese [reconstructed pronunciation])... at least at the start, then I got a bit too recursive and went for Proto-Indo-European, Proto-Semitic and Proto-Sino-Tibetan instead. (It was still "old world-centric" though. In a hindsight I could've used Quechua and Nahuatl.)
It was one of those "I'm going to save the world!" auxlangs, so I made it for the then current year. (This was more than a decade ago.)
The reason why I picked those five languages as vocabulary sources was naive thinking - by including non-European prestige languages, I could make it slightly less Eurocentric; and by using classical languages only, I'd be a bit less biased than by using modern languages as vocab source.
Eventually the conlang evolved (or devolved) into a grammar toy. The "classical vocab" is still there, untouched; but now I can use it to explore a bit more potential features for other conlangs, such as verblessness and the likes.