Why this plugin ? ​

It aims to reproduce vuex-persistedstate for Pinia.

Do I need a plugin to persist my stores ? ​

Short answer: "no". There are a lot of solutions to persist store data in storages such as localStorage that don't even involve injecting a plugin into Pinia. Here's the example with VueUse useLocalStorage :

import { defineStore } from 'pinia'
import { useLocalStorage } from '@vueuse/core'

defineStore('store', () => {
  const someState = useLocalStorage('stored-state', 'initialValue')

  return { someState }
})

import { defineStore } from 'pinia'
import { useLocalStorage } from '@vueuse/core'

defineStore('store', () => {
  const someState = useLocalStorage('stored-state', 'initialValue')

  return { someState }
})

Why would I need this plugin ? ​

The pinia-plugin-persistence plugin aims to provide persistence to Pinia stores for everyone and in every project with a consistent API. Whether you are fine with the defaults to save a complete store, or you need fine grain configuration with multiple storages and a custom serializer, the plugin got you covered, withing the same persist option on the store you want to persist.