If you have being working with AX for a while as an administrator you are probably quite familiar with the procedure of installing an additional AOS. If you are new to AX then this might be something new to learn. You might need to install additional AOSs in your implementation project for a QA system, training etc. In a production environment you will certainly have additional AOSs for load balancing like moving the batch load onto a specific AOS or setting up an AOS for integration traffic, or and additional one to spread the user load.
This video walks through some of the basics of installing a new AOS and getting it running for a user to access it.