[baseonmars]

For me (and the teams I work with) it's to encourage modular thinking - everything is a candidate module that can be extracted, put in source control and shared.

For tests that go beyond small (unit/spec) level we have a project level tests folder because these tests make use of multiple modules and it wouldn't make sense to place them closer to the code.

Someone else in comments mentioned cohesion - that's another way of thinking about it.