Essentially all modern advertising is done algorithmically. The platform takes conversion events (a typical event is "someone fills out a form"), that signal is sent to the platforms, and the platforms use it to serve your ad to other people who may be interested. GDPR as it is means you need opt-in to do this, so it greatly reduces the effectiveness of online ad targeting.
So in practice, say you make a new cool B2B tool for, say, plumbers. It automates your plumbing business and makes plumbers more money.
In the US, you can make a Meta ad campaign with broad targeting and Meta will use algorithmic magic and be able to just find plumbers for you to show your ad to.
In the EU, this doesn't work as well, so its harder to find plumbers to show your ads to. Less plumbers get to use your product as a result. So its just one reason it's hard to get your EU based Plumbing SaaS off the ground.