As @bmike said: port 80 is where just about every non-secure web browser is set to listen for incoming http connections by default. So any communication to unencrypted http services is going to require port 80 to be open.
However, if you open up port 80 for every program, you're going to lose a lot of control over what information is being sent out of your machine (remember, Little Snitch is only an outbound firewall solution).
Little Snitch has the ability to set outbound connection rules on a program by program basis. If you're worried about programs sending out information without your authorization you really shouldn't ever set a general port rule in Little Snitch that allows any program to send out over the port. Instead of allowing all programs the ability to send information out over port 80 you should enable them one at a time. This can be arduous as a lot of programs are going to want to talk over port 80 but it's really the only way to ensure you know who is getting information about you.
When Little Snitch asks if you want to allow or deny a program wanting to send out over a particular port select allow but for this program only. This way you know which programs you've authorized to use the port. If you wanted to get really controlling you could limit each program to the port and the specific address.
You'll basically never want to use the 'Global' tab in the Little Snitch allow dialog:
The tabs across the top of this box control how long the rule persists for:
- Once: Just this one time. After one access, permission is revoked.
- Until Quit: As long as the program stays open. After the process ID of the program changes, permission is revoked.
- Forever: Across all instances of the program for ever more.
- Global: Not just for this program, but any program that wants to use this port.
The list of options control how specific the rule is:
- Any Connection: Allow it to use any port to connect to any destination
- Only port 43 TCP (nicname): Allow to connect to any destination over port 43 using the TCP protocol
- Only whois.arin.net: Allow it to connect to only the destination whois.arin.net over any port using any protocol
- Only whois.arin.net and port 43 TCP (nicname): Allow to connect to only whois.arin.net and only over port 43 using the TCP protocol