For me, a central set of heroes, along with CC, means an attacker needs to spend the time and troops to lure and kill a lot of defenders from the core. My base is pretty big, covering most of the map, so luring them first takes the right kinds of troops, and depending on my exact layout hopefully there are no dead spots / freebies, to help lure them out to the edges.
Against mass hogs, in particular, this can make the difference between being wiped for a 3-star loss, or getting a 1-2 star loss because the attacker spent the time luring and killing everything, but ran out of time to finish the job. That's my recollection from Masters, anyway. In Crystal, a lot of attackers don't bother luring, and end up getting pasted by the clan castle and/or heroes. The odd skilled player throws down some giants, archers and/or wizards, then hogs, and gets away without luring anything first.
Against other armies, central heroes are not always so useful, it depends.
