It's for the pre-heat right enough.
Quite a good idea IMO that you can time it,up to three times per day I believe.
If the boiler was fitted in ,say a bedroom, or a loft with a bedroom below then you wouldn't want the preheat on since the boiler would be constantly trying to keep the domestic hot water warm which might affect your sleep.
I have a Sabre 35 HE and every time I want to wash a small item,cup for example, then the boiler naturally fires up to deliver hot water to the tap however it goes directly to full flame so that is 35 KW output of gas to heat enough water to wash a cup but by the time the water gets warm I've usually turned the tap off because I've washed the cup already so a lot of wasted gas there over the course of a day.
With the pre-heat on the WB as soon as you turn on a hot tap you get enough hot water to wash the cup before the boiler fires however you really don't want the pre-heat on all of the time because that will be a waste of gas too and you don't really want to turn it off either because it is a good feature so the way round that is to time the pre-heat to come on for periods when you would use it,breakfast,lunch and dinner times for example.
BTW, the DT10RF also allows timing of the pre-heat and you can have it in optimising or digistat flavours.
As Asda say "every little helps"
