Co-ops have reserve accounts for large scale systems replacements and emergency repairs. Some portion of these are kept in interest bearing accounts for scheduled replacements and some is kept liquid for emergencies.
There is a document called a replacement reserve study which is a schedule of expenditures on replacements and transfers to finance them.
As a finance chair I am interested in having an account at a credit union that supports the replacement reserve. The controlling contract should be the replacement reserve study. The co-op would make regular deposits, have access to funds as per the replacement schedule, as well as an agreed amount of liquidity for emergencies.
The expected advantages would be a slightly higher interest rate (mostly from spreading the liquidity requirement across many co-ops) and the co-op not having to manage the investments.