Some people know the computational complexity of something they write without knowing it in terms of Big O. Before my CS degree, I understood that looping through a list and for each item, looping through the same list again was not optimal. And someone only needs to write an O(n^3) algorithm once to realize that it's a very bad idea for any but the smallest amounts of data. Knowing about Big O is nice though, if only to understand documentation (realizing that O(nlogn) is better than O(n^2)).