What will happen if you choose not to activate the card and decide to just leave the card aside and not use it forever? You may think that by simply not activating the card the account’s not open, right?
Well... technically, your credit card account is usually opened the moment you’re approved, even if you don’t have the physical card. Sometimes, you’ll have to activate the card once you get it, and the amount of time you have to do so varies by issuer.
As long as the card doesn’t have an annual fee, generally it will not affect your credit scores if you don't use it. But some of these "unapplied" credit cards charges an annual fee, e.g. OCBC's Plus card.
Usually, for security reasons, a credit card usually needs to be activated once you get it in the mail, before you can use it. Even if the physical card is un-activated, the account can still be activated. This can help prevent an unauthorized person from stealing a new credit card from your mailbox and using it. However, activation of the card isn’t the same as opening the account.
When you apply for a credit card, you’re agreeing that you want the account open if you are approved. So, in most cases, your credit card account will be opened automatically upon approval, even before your physical credit card is activated.