Multiple Application Server

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.