If you don't let your new employer know what super fund you are with, your employer needs to check with the ATO whether you have an existing super fund linked to you (a stapled super fund). If QSuper is your only super account, or if you work for a Queensland Government employer, your new employer will still pay your super to your QSuper account.
If you have a QSuper account but you have a different super fund stapled to you, your super would go to that fund.
This could result in you paying multiple fees to multiple funds, and make it harder for you to keep track of your money. If you do have more than one super fund, find out how to combine them.
You can always choose what super fund you want to join, or change funds at any time.