Why do I need authorization in the WEB API?

Question

API

I read and am surprised, double authorization is used everywhere. Most often, a standard, server one, where a token is issued and all work is already done with it.
Here is a site, you send it a POST array and it returns JSON. A classic of any API in fact.
Why can't I send a username and password with every POST request?

Answer 1

Authentication can be more difficult than entering a username and password. Here you want to enter a two-factor through Google authenticator/sms/email, and logging in with a new one every time will not be an option.
Well, store the username and password of the client in clear text on the client all the time the application is running .. Not everyone will like it.

Answer 2

What is token based authentication?