The new server-side Office application features for SharePoint 2013 include the SharePoint Office Web App Server, PowerPoint Automation Services, and Machine Translation Services.
Excel Services includes new capabilities with the notion of Excel everywhere with the innovative Excel Interactive View that can bring any commonplace HTML table to life with a couple lines of code. Also, the new OData access to Excel tables stored on SharePoint can extend the uses of data with Excel spreadsheet to even greater uses. Lastly, you can easily create data-centric Apps for SharePoint using the redesigned Microsoft Access client tooling and Access Services powered by Microsoft SQL server for on-premises and Windows Azure SQL Database for Office 365.
For a developer, these Office and SharePoint application services provide the APIs, technologies, and infrastructure to build innovative and powerful solutions. Your solutions can shape end users’ day-to-day activities and help them to be more productive in what they do, which is what building Apps for SharePoint is about.