For coding reasons, the game is limited to 2 billion total mana in the bank for a player at any one time. Of that, you can reach 600 million (on PC, believe lower on consoles) by selling items to the vendor or earning it through playing rounds, the rest you would have to get through trades or selling stuff to other players in your tavern. If you don't want to go over either of those caps, you can make a mana token to store in your item box. Don't make a token worth more than 600 million mana, as that is the most you would be able to sell it back to the vendor for.
Players who want to trade items valued at more than 600 million, or more than 2 billion, trade multiple mana tokens in exchange for the item, and sell the tokens to the vendor when they want the spending money.
The other option for not going over the cap, and the only one available if you're in the middle of a run, is to upgrade a weapon or armor til you have room for more mana. Some folks keep a junk weapon to upgrade in their box, just to act as a mana sponge til it hits around 600 million, and treat it just like a mana token in trading (I think elemental damage upgrading gives the most end sale value per invested mana, don't hold me to that). That's of course assuming they don't have something else they'd rather upgrade.
TLDR - It's a workaround the developers came up with to deal with a problem they don't want to take the time to code around that puts the hassle on the players instead.