There are a lot of benefits. You can take a look at this post: http://landlordaccounting.com/forum/discussion/346/why-are-classes-and-sub-classes-needed/p1
Also some highlights from there are included here:
Classes are used to slice-and-dice your income statements. Generally people have a class per property (or sub-classes per unit in multi-units). This lets you aggregate reporting for all properties -- or drill down and look ad them individually.
Classes are the standard way QB's uses to split up income/expense among 'categories'. Following this convention goes with the way other businesses split income among business units. Think of the customer/jobs setup as organizational, for convenience. You namespace the tenants (jobs) by the use of customers (properties). Income and Expense are split via classes.