While 'localhost' tells your computer where to send traffic, a tells it which specific program or service on that computer should receive the traffic. Think of localhost as the street address of a large office building and a port number as the individual office suite number. The network data must have both pieces of information to be delivered correctly.
Beyond government services, developers use this port for testing "repacked" or portable applications that require a local server. localhost11501 portable
Malicious websites you visit while the portable server is running could trick your browser into sending requests to localhost:11501 . Modern browsers mitigate this by requiring localhost , but it is still theoretically possible. While 'localhost' tells your computer where to send