For example, I see Amazon.com now is offering a $30 cash back when I first apply it.
However, I have 1 visa card already, I only wanted apply it for that $30 benefit.
If I apply it and I do not use in the future, would it affect my credit score or any other things at all?
(Does this answer applys to all the credit card?)
You get a credit ding for the hard pull they do to check your credit. This wears off after about 6 months.
You get a credit ding of an unknown amount because the new credit card lowers the average age of your open credit lines. This may or may not be important, depending on the age of your other credit.
Extra credit lines are alleged to help your credit score by increasing your available credit, which if you’re carrying a balance will decrease your percent utilization. However, having too much available credit can spook lenders.
If you leave the account unused it will be closed by the issuer. Having a credit line marked closed by the credit issuer on your credit report reportedly damages your credit rating (it may not factor into your score, but might alarm lenders reading through your credit report, since often credit lines are closed by the issuer due to payment problems). However, sometimes they close the account but report it closed according to customer request. This should not adversely affect your score unless you’re carrying balances, thus increasing your credit utilization, or the credit line that got closed was one of your older ones, decreasing your average credit line age.
If you’re not planning on making any major purchases (car, house) in the next 6 months to 1 year it probably won’t do any harm to open the account. You can always keep it active by buying a gift a couple times a year, so the account won’t get closed and will age and benefit your credit score. You can buy all sorts of stuff off Amazon now.
Of course double check to make sure there are no fees, those may end up making it not worth the bother.