Benefits of Private Constructor - Singleton Design Pattern

I understand that it helps creates single copy of class.

I would like to know:

1. What is the harm in creating multiple such instances of a class ? since garbage collection will take care of it.
2. Is it that it helps when multiple classes/threads use it and Java may run out of memory ?
3. It helps avoiding memory leak ?
4. How to decide if singleton pattern is better for me use case ?

Or could you please lead me to resources that can be helpful to learn more about it.

Thanks

See these,

Design Pattern: Singleton Pattern, pros & cons

Singleton Pattern – Positive and Negative Aspects

Singleton