Many years ago when I was painting LOTS and lots of Revell 30YW figures I finished them in gloss varnish, mainly because I didn't trust the paint to stay on. I hate them now, the problem is that I also don't trust Matt Varnish sprays enough to ust give them a squirt all over.
I was told many years ago - even before the 30YW figures, that a coat of gloss, followd by a coat of matt actually makes the colours 'pop' more. But I'm not sure that this applies any more considering the strides we have made in colours and techniques since then.
Ah the good old times when there were just those enamel colours. Still got lots of those and use them on occasion. Way back then I gave my figures a coat of gloss varnish because I had the impression it would protect the tiny parts, bayonets etc. and the colour was less likely to flack of. And I still think it somehow might have protected the older figures from crumbling to dust.