Can someone explain me why we need them?
The question isn't "why have altcoins?", it's "why not?"
And keep in mind that everybody doesn't have to agree, those who think alt coins are bad can ignore them, those that think they are good (or at least interesting) can work with them.
I mean, if every shop adopt bit as payment will be expensive to adapt other coins as payment also.
This would only be true if every business was going to setup their own wallet and daemon, etc, which no business is going to do, they're all going to use 3rd party services. 3rd party services can add as many coins as they like and it makes no difference to the business.
For the coin payment gateway the effort in adding another coin is minimal, (cryptsy supports about 50 coins).
For a business using the gateway they wouldn't even notice when a new coin was added, their customers would just have 1 extra option when making a payment.