SharePoint Office Web Access

SharePoint capability of creating/viewing office documents in the browser itself although not all Office features are supported in Office Web Access.

Governance purpose you can enable Office Web Access only on some site collections as required to keep the Server Performance under check.

All About Office Web Access:

http://technet.microsoft.com/en-us/library/ff431682(v=office.14).aspx

Installation Consideration to manage services and service application running in the SP Farm.

You need to differentiate between services and a service application. A service is a component that provides an output that can be utilized by an application. A service application is an application that is built to utilize one or more services that exist in the environment. Services in SharePoint 2010 are the foundation for service applications. Some of these services are associated with service applications. You deploy your service applications by starting the associated services on the desired server, selecting those same services when running the initial Configuration Wizard, or by using Windows PowerShell. The number of services running varies depending on the business requirements of your environment.

So for the Office Web App

We can have the services running (to support office web app i.e. Word/Excel Services) only on one Web Front End server and a separate server running service application (for office web app) to optimize performance as OWA eats ups lot of server resources compared to other features of SharePoint