Why is “Access this computer from the network” permission required for proxy credential?
Because if the account doesn't have the &quot;Access this computer from the network&quot; privilege your job step will fail with

Message Unable to start execution of step 1 (reason: Error
authenticating proxy xxxx\xxx, system error: Logon failure: the user
has not been granted the requested logon type at this computer.
('Access this computer from network')).  The step failed.

Ok, followup question, why is that?  I don't know, but either SQL Agent is performing the impersonation using a call to LogonUser with a dwLogonType that requires this privilege, or SQL Agent is checking the privilege directly and generating this error message, instead of just trying to perform the job step and reporting a failure.
In both cases, this suggests a security feature in SQL Agent.
IE if you don't have the privilege to connect remotely to this computer, but you do have the ability to connect to to SQL Server and create a job, then you could bypass the bar on remote authentication when SQL Agent logs you on locally to run your job step.