18+ Oyster card discount. Can you explain it better?

I'm a bit confused with it. I've already got the card, but I'm not sure if there is any point for me to get a weekly or monthly travel card if I travel for less than 7 days, or does it not matter. Let's say I got a 7 day travel card...and even if I have a discount, if I only travel 5 days, I'd be losing money? Do I understand this correctly?