There are 2 parts to the answer:
1. General info about use of ports < 1024.
In the dim early ages of TCP/IP, it was thought that one way to make systems more secure was to place the "dangerous" services in a zone where only an administrative (root) user could use them. Thus, in many OS's, if I found that the main webserver on port 80 wasn't up, I could try, as an ordinary user, to startup a bogus webserver, but my bogus server wouldn't be allowed to start because access to port 80 was denied.
A related condition is that since many servers operate in a many-clients/one-server mode, the server port number is fixed so that it can be easily found, but each client is using a (high-numbered) dynamically-assigned port ID. So these ports can be assigned attributes more appropriate to clients.
2. The whole concept of which port number to use is based on convention. When you specify a URL beginning with "http:", for example, your computer's resolver knows that unless you specifically supply a port number, Port 80 is the port you mean. This is known as a "Well-Known Port Number", and you can find lists of them with any Internet search engine. The Well-Known port is just a convention, however, and (subject to the <1024) rule and whatever firewalls may be in the way) actually any port number can be used.
Conventionally, while the main http port is port 80, certain alternative ports are common for secondary servers and
test servers. For HTTP, ports 8000, 8008, 8080, and 8180 are popular. Since Tomcat is often paired with Apache, a common configuration is to have Tomcat serve HTTP on port 8080 or 8180 for test and debug purposes, but have Apache forward production requests which were made through port 80.
Note that Tomcat, like many servers, is not limited to using only one port. You can add and/or change ports and services in the Tomcat server.xml file.