With the sorting solution, you can stream the result directly to a file _without_ holding the entire file in memory. So it can be better if you are memory const

With the sorting solution, you can stream the result directly to a file _without_ holding the entire file in memory. So it can be better if you are memory constrained (e.g. if the files are bigger than what you can hold in memory), or if you want to hold the working set inside the CPU L3 cache as 'cperciva is alluding to.