On high-impact systems, if you look at AI law in different parts of the world, you see the governance applies to systems that are likely to cause a significant risk of harm.
There are lots of AI systems that we are using every day, like our GPS, that do not have a significant risk of harm to an individual, thus they should not be subject to the kind of governance oversight that we're talking about in AIDA.
The importance of “high-impact” is that it is a trigger to determine when governance is required, as stipulated in the law.