Just because I have heard of NordVPN doesn’t mean I’ll necessarily use it (in fact I use arch mullvad, btw.)

No it does not mean you will pick it. It means you are more likely to pick it. Given all else being equal you are vastly more likely to pick something familiar than something unfamiliar. And it all comes down to trends and statistics. The hope is that more people will go for your brand that leads to more sales then the cost of the marketing in the first place. You might not go for NordVPN for other reasons, but can you say that about every product you have been advertised to? If anything the more you know about a product the less advertising will affect you in the familiarity sense - these adverts are not so much meant for you as they are for people not familiar with VPNs at all.

But there are a lot of studies on the topic like this and this meta analysis that seem to conclude that advertising is effective. And there are a lot of studies on what various aspects of adverts make them more effective. I am yet to see any research that says adverts are ineffective overall, though I have not dug that deeply into it.