What is an altar / shrine for?

While one is recommended, it is Not a requirement. 

1. It provides a mental benefit (Knowing this is “my” quiet space to practice).
2. Acts as a workstation. Things have memory, places have emotions, feelings, etc attached to them. Having a private, controlled space to do your cultivation practice is helpful.
3. Designating a spot where others know to minimize distractions.
4. A spot to store your physical tools for practice.

* There is a specific way to set it up
* It can be charged with positive energy like a battery, in case you need it when you are low
* It is not meant for worship of a god, it is for You and Your inner cultivation

"Recommended but NOT a Requirement"

The altar is for YOU, not for anything outside of You. What is inside you is bigger than anything outside. However, it is always good for a painter to have a space with an easel and a brush.