What is the purpose of the bank?

[Brodi]

I'm confused as it seems there is no downside to just packing your coins from level to level.

[queb]

If you have excess coins and don't need to spend them you can store em in a bank. Your bank account will save across runs so if you start a new run and find a bank you can start off with a ton of coins. It's actually a really powerful mechanic

[Vividnightmare]

If the bank ever appears again.

[ethereal]

It's also nice if you're low on health and don't think you're going to make it much longer.

[Sailor.Goon!]

Also worth mentioning that once the bank is given a certain amount of gold (mine started at over 100) a wall will occasionally open into the back which contains a (free) chest with additional gold in it. Definitely worth the long-term investment.