People use the cable ties when they are using older luggage, that aren't equipped with TSA approved locks (if they are TSA approved locks, they can open them with their keys) If they do open your luggage they will leave a card saying it has been opened.
The cable ties used on older luggage, without those locks, are so you can leave the luggage unlocked, but with the ties to secure them. If the TSA does want to check your luggage, they will cut the ties. But won't have to break your lock.
You certainly don't need to use cable ties AND to lock your luggage, if it has the TSA approved locks.
Of course this applies only to airports, if you're flying. At the port you can lock your luggage before checking it, as no one will open it without you present.