Golang中单例模式的应用场景有哪些？

package config

import (
    "github.com/spf13/viper"
    "sync"
)

type Config struct {
    DatabaseUsername string
    DatabasePassword string
}

var instance *Config
var once sync.Once

func GetConfig() *Config {
    once.Do(func() {
        viper.SetConfigFile("config.yaml")
        viper.ReadInConfig()

        instance = &Config{
            DatabaseUsername: viper.GetString("database.username"),
            DatabasePassword: viper.GetString("database.password"),
        }
    })
    return instance
}