There's multiple cases out there that don't offer 160mm or better motherboard clearance for an aircooler, but will mount a 120/240/360mm radiator. So if wanting to get a bigger cpu, add some OC and still have cooling capability over or above the cpu output, often a larger rad is the only alternative. Considering a decent, full custom loop can easily top $900 and requires greater periodic maintenance, fluid top-offs, fluid changes etc, an AIO fills the niche between what's cost effective and available when big-air simply is not an option.