Pls explain how early summoning spells are useful

[merlinme]

Well, maybe. It's true that Nature doesn't seem to have any really game winning summoning spells at the high levels, so maybe 2 Vine Ogres a turn for two gems is a good use of your average mage's time. A very large army of troops which don't rout and don't eat is obviously a good thing. I'd still prefer the Summon Horde as more bang for your buck in the short term, though.