Paying for more space get really expensive as you go and RAID players may not have enough silver or may not want to use up their silver at a specific moment because they are preparing for an Artifact Enhancement event or if they are focused on other objectives than silver income.
You can avoid (or postpone) paying millions of silver for Extending Champion Collection by storing champions in the Inbox, where they can stay up to 99 days. See below a picture I prepared to show you my "food" champions. I pick them up from the Inbox when I need thereby avoiding to pay the silver costs.
You plan for this by not ranking up champions while you farm campaigns. This way you don't free up space and all additional champions go to the Inbox. When you are finished farming or when you need to free up space for shard pulling then you rank up the champions you have prepared and free up the space you need while still having lots of champions ready in your Inbox. (Naturally plan ahead and rank up during a Champion Training event that is right before the Summon Rush or Champion Chase)
For the daily quest to summon 3 champions you only open up 3 spaces which get immediately filled up by doing the quest.
To give you an example currently the expansion would cost me half the silver I have and since I am focused on other goals I don't invest energy to run Spider for silver income and this method allows me to manage my champions without spending the silver.
