Like all politically-charged terms, the original meaning has been long lost as the context in which it was coined is forgotten.

I think the term ‘virtue signalling’ was originally intended to point out a perceived hypocrisy - that it’s much easier to gain public support for an idea/cause/campaign if that campaign is perceived to be helping some disenfranchised group - even if the campaign also benefits the organizer, and even if the campaign is not necessarily wanted by some or even all of the allegedly-aggrieved group.

It did start out that way (an attack by conservatives against progressives) but has since become ubiquitous. It just refers to immaterial forms of protest that don't accomplish anything except signaling support for a cause.