It's a hang-over from WW1 - so kids could get back from school and pick the crops from the fields in the evenings.
To be honest, it used to be a nightmare when I worked in the MoD. For around a fortnight every spring and autumn we could only talk to Germany for about 30 minutes in the morning and 30 minutes in the afternoon, as at least one of us was on some sort of tea-break or lunch-break.