@
athy_nguyen,
To put it in prospective and in Leman's term, the reason you need a domain name is to provide a name that is easy to remember or you can easily associate it with.
Your ISP provides you with an IP that is the equivalent of your home address.
If we are talking about your own home address, for any member of your family, or friends or official services, for them to know how to send you mails or get in touch with you, you would have your name and address registered with the post office. This way, when someone write you a letter or send you a parcel, the post office will be able to direct the mail to your home.
When you look at it from the point of view of the internet, you want people to reach you by using an address that is easy to remember. Unfortunately, your ISP will only provide you with an IP address. The problem is that the IP address could be leased to you (you are not really aware of it) for a certain amount of time, called lease, and will be renewed at any time, unless you have the ability of having a fixed IP address.
Regardless, you do not want your friends having to remember you IP address, instead you want them to use something that is easily remembered.
For that purpose, and to ease on finding your address, you would want to register a domain name and associate it with your ISP IP address at the time it is allocated to you.
One the address is processed by the post office, it will be propagated to every single post office in the country.
Only then will you be able to tell someone to send correspondance to that address.
If the address is non existent at the time, any letter that would have been sent to, would be lost and end up on someones else address.
Registering the address is just garanteeing/informing the system where you live.
The same principle applies to your ISP provider.
Does it make sense?
BTW, your port formwarding would be equivalent to having an room at a hotel for which your address would be associated with a room number. The problem is that the room number is not part of the address.
The way it would work, is that the hotel staff will make a note that says that if you receive mail at the hotel address, the staff, upon seeing your name will now they will need to put it in your income mailbox corresponding to your hotel room.
But of course you will have to let them know you are OK receiving emails and have it placed in your mailbox, This is what the equivalent of opening a port would be. If you say that the port is closed, then the hotel staff will not allow the mail to be placed in your mailbox and could simply be rejected and thrown away. You never hear of it.