Investments properties lead to safe rewards when held over the long term. Anything which makes it easier to hold on to a property is great, property managers make it easy to obtain and hold on to good tenants. Realistically if its easy, you might be inclined to get more properties.
At the start
One of the main areas where they are useful is even before tenant selection. Buying a property is a lot safer with local knowledge of the area, rental managers can provide this. They will generally do reference and credit checks when checking the suitability of a tenant.
Ongoing
Property inspections should be done every quarter usually and will ensure the property is well maintained. A good property manager will also get involved if things start to turn sour by serving relevant notices quickly and representing you at any mediation.
Not all property managers are prefect and you can change the manager mid tenant if you find their service lacking.