The higher end (and more expensive 240mm and larger) can offer really good cooling as well compact form factors. And some cases like the NZXT H1 will only work with a 120mm AIO although compact air coolers are getting quiet good.
A traditional air cooler offers a much lower cost to cooling ratio, with quality coolers from Noctua, Phanteks and bequiet usually beating the cheaper AIOs on the market in cooling performance. Air coolers also dont leak which is rare but still happen nor do they lose performance with age like many AIOs (the fluid can gunk up, bacteria can grow etc, basically AIOs have about a 5 year life span)