Anchors for booster seats are only for convenience, not for safety.
In actuality, the LATCH system, as mandated in US vehicles from 2002 onward, supports a total combined weight of both child and seat of no more than 65 pounds. I found this information on Consumer Reports.
Depending on the weight of your car seat or booster seat, this may mean the LATCH system only supports children weighing anywhere from 32-50 pounds. In these cases, it's necessarily safer to use the seat belts to secure the seat, rather than the LATCH system.
The reason LATCH was developed was not because it is inherently safer, but because it's inherently easier to secure the seats properly.
Securing seats properly with the seat belt straps is more difficult, so more care must be taken in doing so. However, seat belts do not have the surprisingly low maximum supported weight that LATCH systems do. Seat belts are designed to safely secure adult passengers of various weights. For most five-point harness seats, it's also possible to secure them with the seat belt, instead of the anchors.
Booster seats that only use the seat belt to secure the child, and don't secure the seat, gain no additional safety benefit from the LATCH system. It's there mainly for convenience: the child can get in the seat themselves without it shifting and it doesn't become "free" when not in use. Children in this types of seats are more likely to outgrow or already by over the weight limits of LATCH systems.
As Byran answered, if the booster seat is not in use, it should be anchored with the LATCH system or stored in a safe area, in order to prevent it from becoming a hazard during a car accident.
Answer adapted from another answer of mine.