To varnish or not to varnish. Lots of opinions. I don’t varnish my acrylic paintings. I didn’t varnish my oils either but that was mostly laziness. With acrylics the finished product looks fine without varnish. I know the paint is slightly porous and tacky but not much. There is a good argument here for an isolation layer that can’t be removed (at least not easily). The best reason for varnishing that I have heard is to standardise the surface finish over the entire painting. Acrylic paint is quite matt and the colours will often benefit from a little shine. I like the matt surface finish of acrylic paint and I don’t see a need to change it.
The best reason I’ve heard not to varnish is that ALL of the manufacturers recommend that paintings should be varnished. This makes me suspicious since they have a vested interest in selling product.