Many observations of the sun's corona just use a disk attached to the telescope, as you're suggesting [0]. However, for Proba-3 they want the disk to be over 100m away from the telescope, which isn't practical. Also, one of the goals of Proba-3 is to develop formation flying technology, because it will be useful for many other missions in the future.
[0] https://en.wikipedia.org/wiki/Coronagraph