When an opponent has opened the bidding, you may still want to bid, perhaps to disturb their auction.
This is why you can overcall with fewer points than are needed for an opening bid. But remember, when you overcall a suit at the 1-level, you always need to have at least five cards in the suit.
You can overcall with as few as 8 points, but if you are weak, your suit needs to be nice, to give your partner a good idea of what to lead.
Remember: an overcall at the 1-level shows 8 to 17 HCP and a five-card suit.