The way you handle rent collection and late payments can be the difference between success and failure as a landlord. Collecting rent on time every month is the only way to maintain consistent cash-flow, and your tenants need to understand this is not negotiable. By hiring a property manager, you put a buffer between yourself and the tenant, and allow them to be the bad guy who has to listen to excuses, chase down rent, and when necessary, evict the person living in your property.
If you let them, your tenants will walk all over you. They have to be trained to follow every part of the lease or deal with the consequences. Property managers have an advantage because tenants realize that they, unlike the owner, are only doing their job and are obligated to enforce the lease terms. Many property managers will tell you that it is considerably easier to manage other people's units rather than their own for this reason.
Regarding evictions, there are strict laws concerning the eviction process, and doing it wrong, or trying to evict a "professional tenant" can be a MAJOR fiasco. A good property management firm knows the law and has a good process for obtaining the best possible outcome given the circumstances. Never having to handle another eviction can be a compelling reason to consider hiring a property management company.