What if I don't have [::] in my nginx config file?

Question

My nginx config looks like this:

server {
    listen 80 default_server;
    index index.html;
}

Must I add listen [::]:80 default_server; or is it not mandatory?

So the config will end up looking like this:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
index index.html;

}

I found that it is for IPv6 configs, but I am not sure, I understand what does that mean in this context/config file. Could you elaborate more? Sorry, for noob question.