Unable to understand precaution for SSH key for Git

I am setting up Git for Github on my ubuntu and decided to do the SSH instead of HTTPS way. So, I am at the step of Creating SSH key (my .ssh folder is empty). Now, I came across this precaution on Github when I searched why I need a passphrase.

With SSH keys, if someone gains access to your computer, they also gain access to every system that uses that key.

What does it mean? What I intend to do is to use this key only for Github.

As far as I understand, it would mean that If I use this SSH key for any service (Github and some other also), then compromising the key would result in the access of all those services that are connected by this key. Right?

I am actually ambigous about the gain access to every system that uses that key line. I don't know much about SSH keys and their purpose but avoiding HTTPS as I would have to write my Github PAT everytime I connect.

Thanks in advance.

Yes, that sounds right. Stealing a SSH key is fundamentally no different from stealing a password – if someone finds your SSH private key file (~/.ssh/id_rsa) they can use it to access your account on any system that you've configured to accept that specific key on.

But, as SSH keys are considered "stronger" than passwords, it is much more likely that someone will generate a keypair once and will use the same keypair everywhere, for decades.

(SSH is not just for Git – it's how nearly all non-Windows systems are remotely managed, and even some Windows systems too. While there is at least some understanding about password reuse now, probably a lot of sysadmins or developers use the same keypair to access their GitHub, their work servers, and their home PC/NAS/RPi as well.)

This often makes the SSH keypair a more valuable target than a password, and there have already been various occurences of malware which steals ~/.ssh (in addition to stealing wallet.dat and such). Hence the precaution.

So the passphrase is simply used to encrypt the private key file on disk (exactly like how password manager apps have a "master passphrase" that encrypts the password database). It is not mandatory – you decide whether the risk is acceptable – but it's highly recommended. As the encryption is entirely local, you can always change (or add/remove) the passphrase using ssh-keygen -p, without needing to generate a new key.

(Also, if you're using the full desktop version of Ubuntu (i.e. not WSL), most likely you have ssh-agent automatically set up through GNOME Keyring, which will remember the SSH key's passphrase in the "keyring" that's protected by your system password. There are some risks associated with that, too (any app can read the keyring) but at least it does keep the file encrypted on disk, protecting it e.g. if it's on a laptop which gets stolen, while still giving you the convenience of password-less logins.)