Minutes and seconds being groups of 60 for the next unit up means you can divide an hour/minute into halves, thirds, quaters, fifths, sixths, etc etc really easy without breaking out a calculator. The day divides pretty easily too cause 24 has a lot of factors. I guess in this age of computers it doesn't really matter anymore and it's just pure inertia that pushes us along, although every once in a while you get a confused person who thinks an hour is 100 minutes or whatever cause they never really thought about it.