American way of counting

One Mississipi, Two Mississipi, Three Mississipi, ...

Have you ever seen anyone who counts like "One Mississipi, Two Mississipi, Three Mississipi .."? Why do they count like this? The basic idea here is that saying "Mississipi" takes up about a second. So, you can count seconds by saying like that way though it is not exact time but approximation. There is a similar counting when you try to sleep. One sheep, two sheep, ...  I don't believe it is to measure some amount of time.