The stated reason in "Dreadnought" is that Tirpitz wanted the purchase and construction done in groups of 8 so that the politicans in the Reichstag could not interfere in their construction from year to year. The ulimate idea was the purchase of two 8 ship battleship squadrons, with a flag ship and at least 4 in reserve.
"It was a perfect attack, Right Height, Right Range, Right cloud cover, Right speed,
Wrong f@%king ship!" Commander Stewart-Moore (HMS Ark Royal)