Secondly, port forwarding exists for security. Problem is that it is tied to your home router, and if your home router connects multiple computers to the internet, they are all tied behind one IP, if someone sends something to your IP, it does not know what computer to send it to, unless you tell it what computer to send it to, through port forwarding.
Your home network for example, is behind one IP address, which is basically your internet address and how stuff knows how to find you on the internet.
Firstly, it exists so that people can initiate connections to a network that has multiple computers. But if you port forward, they can be the one initiating the interaction with your computer.
Basically, when you have no ports open, you can only be the one initiating the connection, though after that initiation, the other computer you initiated the connection with can reply back to you. When you connect to someone on the internet, you do so through their open port, and ports generally have different uses, such as for most websites, it is either port 80 or port 443 (Reddit is port 443).
Port forwarding would be the equivalent of opening a window into your house, but it is all right, all the doors from the room that the window was opened in is locked anyways, so if someone climbs in, they can't do anything malicious to the rest of the house. Ports are the windows and doors to your house.