Multiple Application Server

Question

I am novice in SharePoint. Let me ask the reason why there is a need of multiple (specifically four) application server in a SharePoint farm. Actually we have a client and this is their requirements:

4 Web Server (WFE)
4 Application Server (I have no idea for the function or advantage of multiple App Server)
2 SMTP Server
2 Workflow Manager Server (What is the advantage of having 2 workflow manager server)
2 SQL Server (AlwaysOn cluster)
1 Integration Server (I have no idea what is Integration Server)

I hope that you can enlighten me Mam/Sir on this.

Answer 1

Application severs are the heavy lifting servers, and host the service applications, including the search, user profile, performancepoint, excel, and other resource-intensive services. Having four application servers may be reflective of the amount of users that will be utilizing these services.

Workflow 2013 is new to SharePoint 2013 and allows users to scale out Workflow 2013 without the bind of SharePoint, having two servers for Workflow 2013 is basically two servers that are running the logic of all the SharePoint 2013 workflows (not the sharepoint 2010 version workflows)

Integration server may be the Office Web Apps server. This a server that provides a web-based Office functionality to users without having the need to open documents in the client version of Office. They have the ability to view and edit with Word, Excel, PowerPoint, and OneNote, and also the ability to coauthor in the browser with Word OneNote and PowerPoint