What is the SSH key passphrase used for?

Here's the answer I was looking for myself, and could not find until I experimented with both having and not having a passphrase: When the private key is encrypted, then you have to enter the SSH key passphrase every time you need to use it. Then I learned about the "ssh-agent" daemon/service that can cache the credentials once per login. When I went through this, I was establishing my Gitlab.com connection.

Here is the script of my trying both.

Microsoft Windows [Version 10.0.19042.985]
(c) Microsoft Corporation. All rights reserved.
C:UsersAdministrator>where ssh-keygen
C:WindowsSystem32OpenSSHssh-keygen.exe
C:UsersAdministrator>ssh-keygen -t rsa -b 2048 -C "XXXXXXXXXXXX"
Generating public/private rsa key pair.
Enter file in which to save the key (C:UsersAdministrator/.ssh/id_rsa):
Created directory 'C:UsersAdministrator/.ssh'.
Enter passphrase (empty for no passphrase):           <=====  here I did NOT use a passphrase
Enter same passphrase again:
Your identification has been saved in C:UsersAdministrator/.ssh/id_rsa.
Your public key has been saved in C:UsersAdministrator/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:------------------------------------------- XXXXXXXXXXXX
The key's randomart image is:
+---[RSA 2048]----+
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
|  xxxxxxxxxxxxx  |
+----[SHA256]-----+
C:UsersAdministrator>cat %userprofile%/.ssh/id_rsa.pub | clip
    NOTE:
    NOTE:   at this point I navigated to gitlab in my browser, and pasted the key
    NOTE:
C:UsersAdministrator>ssh -T git@gitlab.com
The authenticity of host 'gitlab.com (172.65.251.78)' can't be established.
ECDSA key fingerprint is SHA256:-------------------------------------------.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'gitlab.com,172.65.251.78' (ECDSA) to the list of known hosts.
Welcome to GitLab, @!
C:UsersAdministrator>ssh -T git@gitlab.com
Welcome to GitLab, @!
C:UsersAdministrator>ssh-keygen -p
Enter file in which the key is (C:UsersAdministrator/.ssh/id_rsa):
Key has comment 'XXXXXXXXXXXX'
Enter new passphrase (empty for no passphrase):       <=====  here I DID use a passphrase
Enter same passphrase again:
Your identification has been saved with the new passphrase.
    NOTE:
    NOTE:   now I'm prompted every time
    NOTE:
C:UsersAdministrator>ssh -T git@gitlab.com
Enter passphrase for key 'C:UsersAdministrator/.ssh/id_rsa':
Welcome to GitLab, @!
C:UsersAdministrator>ssh -T git@gitlab.com
Enter passphrase for key 'C:UsersAdministrator/.ssh/id_rsa':
Welcome to GitLab, @!

    NOTE: set up the service to run (it is disabled by default)

    NOTE:
C:UsersAdministrator>sc config ssh-agent start= auto
[SC] ChangeServiceConfig SUCCESS