Because the role of target man is a specialist position - much like being a defender is a specialist position, you can throw Egan up there to ‘win a few headers’ but that is last resort. Madine is a target man, which means not just winning headers, it means linking up the play from midfield to attack, it means taking up good attacking positions, it means being able to bully a defender and be a good finisher - Egan is a brilliant defender but could not do this.
He did, several times actually, and broke through the middle, only to over hit the pass out of play, he played a few other nice passes albeit fairly straightforward ones and was generally looking to move play forward.
My overall opinion is that both Madine and Dowell are good additions.