One reason for doing it is that some airfields only have precision approach aids on 1 runway, the non-precision aid on the other would have a decision height above cloud base!
Another is that winds of 5 knots or so can be quite variable in direction - with the amount of traffic stacked up on approach to LHR can you imagine the chaos if everyone executed a go around and demanded a runway change every 5 mins as the "variable, less than 5 knots" wind changed direction!!!
I know some small jets have a tailwind limit of 10 kts - perhaps someone else can give some accurate figures for different types.