it's because on a pure you can wait for a target without a summon pouch on you, and you'll get a target that is actually your combat level.
Then you can bank and get a summon familiar to use in the fight, like say a war tortoise.. It holds what? 18 extra food?
So lets say you were a pure lvl 70, without the tort you get a lv 70 target, then you bank and get it making you lv 79 or so.
Even though you're lv 79, you can still attack your lv 70 target in 1deep because he's your target.
So basically it allows you to have 18 extra food in a target fight.