Of course it's speculative booking. It removes the seat from inventory - in FareLock's case for 72 hours to a week. The fee varies, but is nominal. Online travel agents popularized an even more prodigal Hold For Free option, though WN doesn't play in their sandbox.
The reservation is created, the flights are reserved for you and the space is being held, meaning no one else can have “your” space when you have a reservation on hold.
Gives you time to transfer miles or price shop. Small price to pay if your plans aren't certain or you think there may be a fare war leading to even lower fares.