Schedule post in WordPress is its inbuilt feature, you don’t need any extra plugin for it, which is great. and if you want the more extra feature then you go with Plugins.
You will find many free plugins for this purpose.
Why does anyone need to schedule their posts?
If you manage to write 4,5, or more post in a day and you don’t want to publish all of them at that time. The solution for it is either save them to Draft and click on Publish Button for publishing when the right time comes. But you need to login to your Dashboard every time when you want to publish it.
The next case, if you completed your post at night and don’t want to publish it right away. You want to publish it in near future.