Why use another person Docker Image?

I’m learning about Docker architecture.

I know that images are made to run applications in containers (virtualization). One thing that I stepped on was that there is an entire community hub for posting images. But what is actually the point of doing that?

Isn’t the idea of images contain a very specific enviroment with very specific configurations that runs very specific applications?

NebulaGraph database: What is the appropriate setting for replica_factor?

Can I set replica_factor as any number in CREATE SPACE statements?
Is it better to set this parameter higher the more machines we use for storage?

The NebulaGraph documentation describes this parameter as follows:
Specifies the number of replicas in the cluster. The suggested number is 3 in a production environment and 1 in a test environment. The replica number must be an odd number for the need of quorum-based voting. The default value is 1.

But I don’t understand why.