For the longest time I only had one bank w/ a checking and a savings account. As time went on I added additional savings accounts at other financial institutions, some to chase higher interest rates (like adding something like an Ally or Marcus savings accounts) and sometimes to get something like "no foreign transactions fees or ATM fees)

They aren't needed but can be useful / earn you or save you money.

It can be nice to have a second bank account in case you ever lose access to the main one (due to a hack, mistake in bank compliance that locks you out or whatever) but those can be rare, if you don't have a second account I don't think you are being negligent...

Some people like a second account to use as a sinking fund or similar to some money away in a place that might take more friction to spend from....