My little 18" Kenmore (built by Frigidaire) does this as well. Happens after the main wash, before the final rinse and before every other wash cycle (that is before the second wash in "Pots & Pans" and main wash in "Normal".
Always thought the purpose of filling and draining was to flush out any muck from the bottom of the tub,and pump so to have fresh for the next cycle. Comes in especially handy when using Electrasol tablets, which tend to be rather frothy during the wash.
L.