If you make your own tofu, the liquid can be reserved and used in things like bread dough, etc.
Store-bought tofu, on the other hand, sits in water (sometimes in an open bin), and the water can be very unsanitary. This type of tofu liquid should be discarded.
All tofu (except tetrapacked silken) should be kept in the fridge, in a closed container, submerged in water (the tofu, not the container!) This water should be changed daily to keep the tofu at omptimum freshness. Even tetrapacked silken (shelf-stable boxed) should be stored this way after the box has been opened.