A block/tenant association is an association of tenants who want to work together to protect their own interests. Much like a union, tenants may band together to implement certain changes, such as getting maintenance and repairs completed in their units, and defending their own legal rights. Because tenants may need to collect money, pay legal fees, or even hire employees, a block/tenant association may need its own EIN.
Tenant associations are often arranged by a single person or a few motivated individuals, and then formed to give tenants a stronger bargaining position. It will usually work as a non-profit organization, and as a non-profit organization, it will be able to bargain and negotiate on behalf of the tenants who are involved. As a non-profit, it will also need to apply to the IRS for non-profit and tax-exempt status.