i think for some security purposes. if you have multiple wallets and one of those have been hacked for any or undistinguised reasons, not all of your savings will be taken because not all your savings is in there. i think thats the reason.
Yes, 100% the main purpose are increasing the security measures of their funds and keep them in some part of the storage and anytime you will be got lost in one of your bucket and the another bucket are still secured.