Sketches will not hide in item panel

I tried projecting a shape to a new plane but was stuck with an unusable sketch, (which I still don’t understand why), I finally created a new body from an extrusion of the old body shape. I then created a sketch on a plane to subtract from the extruded body. Once subtracted, I tried to hide the sketches in the item panel and they refused to hide. I tried to merge the history as I did all the fiddling I needed to and it deletes the body. I’m stuck, frustrated and need some help. Thanks.

Upadate: so I found out it was because it was isolated. I understand, but don’t understand why. It would help if anyone can explain it to me. Also the unusable projected sketch doesn’t make sense.