Hi Felix, also having that edge there actually increases model accuracy because with that edge that you don't like in place it means that this area of the model here:
is able to be represented with a simple analytic plane surface rather than just a generic NURBS curved surface as it would have to be if that whole cutout piece was one larger surface.
With it segmented as it is currently, many CAD/CAM programs will be able to recognize that face is a plane and be able to do special case processing on it when doing various kinds of operations involving it like intersection calculations and projections.
When programs get to use special case processing for intersections it helps get both lighter and more precisely accurate results...
So your result there is good - there isn't any need to try and remove that edge - doing extra work to remove it is more likely to degrade your model a little rather than improve it.
Now, one reason why it's good to have it in your particular case here is that it maps nicely to a large planar region of your model - having a large planar region be constructed as a plane that is bounded by edges is good.
But that doesn't mean that having more edges is always good - there are other cases where it can be good to work on removing edges - like if 2 coplanar pieces are going to end up adjacent to one another, it's better in that case to have one large plane instead of 2 adjacent fragments - that's why boolean union merges those kinds of coplanar fragments together.
It's also not so good to have things split up into a lot of little tiny slivery fragments - say for example your cutting piece was a more arbitrary wiggly thing like a sine wave rather than having one piece that was a line, then for something like that it would be likely be better for it to be one long surface without any internal edges in it.
So maybe the part that's a bit confusing is that there isn't one single answer as to what is best - if you have plane that's next to a curved piece like in your last example, it's good for that case to have those as separate faces so the plane can be a simple analytic piece. But then in other cases it can be good to reduce internal edges though too.
It kind of depends on the particular situation, and it can even depend on what you specifically plan to do with the model....
In your particular case that you posted, I can't see any reason for concern about it though, having that segmented with the plane as its own separate face is most likely the best structure for that particular object.
Hope this helps explain things in some more detail! :)
- Michael