Do I have to create a new branch when I use PR on github?

after looking at the materials on the Internet, it is generally said that it is necessary to build a new branch when PR. Here I have two questions.
1. Why do you want to create a new branch PR?? Can"t I just change it on master and then push and then PR?
2. If I create a new branch master2, locally, can I push to master on the branch of master2?
when I push, if the local and remote repositories are not the same branch, I will prompt

remote: Invalid username or password.
fatal: Authentication failed for" https://github.com/Eve-1995/a."

Mar.04,2021

generally, master belongs to other branches of merge and is used for distribution and is not modified directly on master.
new features and bug fixes are all separate branches.
other branches cannot be directly push to master, need to push and then merge, or in your local merge, push to the main branch.

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