How could it be a better bitcoin if it will be named as a different coin, so what will happen to those people who invested on bitcoin on the first place if the fork became successful that lead to a "better" bitcoin?
It is not. HabBear is a known spammer that tried to infiltrated the Collectibles section once, but people caught up to him pretty quickly.
So if a fork was successful that only means that "bitcoin" will be an alt coin right?
No. Regardless of the success of a contentious fork, it can never be Bitcoin. It will always remain an altcoin.
What benefit can a fork offer? What I understand is that a fork is like an ICO without offering it but it came from the records of BTC and a new is created, so which one should be considered as an alt coin?
You are confusing:
a) Contentious vs. consensus forks.
b) Hard forks vs. soft forks.
Tl;dr explanation: consensus based soft and hard forks can improve Bitcoin. Any kind of contentious hard fork is not Bitcoin nor an improvement of any kind. It creates an altcoin, and altcoin features aren't "bitcoin improvements".