Many people enjoy setting up savings for goals, as well as putting hard caps on the amount of gold they are physically carrying to prevent accidentally bidding over intended amounts.
While I haven't seen it much lately, it had been very common to try and "trick" people into bidding accidentally, larger amounts than is reasonable for items, and quickly passing before that person had a chance to leave party. Having the ability to "store" excess gold would help prevent against situations like that.
That being said, it's just their suggestion, you don't really have to agree or disagree with it.
Plenty of other games have this as a general built in function, and given that we have things in game like clan depot for being a clan "bank" of resources, it wouldn't be that hard of a system to implement.