There are a host of reasons to use a reef ready tank as opposed to a hang on back overflow.
1 - The #1 reason as stated above is the occurance of flooding due to failing siphons. This happens most frequently during power outages. Some external HOB overflows are more reliable than others, but none compare to a reef ready drilled tank.
2 - Additional water volume in sump helps to create more stability in your system. More volume = more stability or as commonly stated in the hobby, 'the solution to pollution is dilution'.
3 - A sump is a great place to hide your heater, temperature and pH probes, a place to put your media reactors, and a place to put your protein skimmer.
4 - A sump can also incorporate a refugium to help export nutrients.
5 - Adds to the overall 'clean appearance' of your system.
HTH,
Russ