Why has going into hiding become such a pressing matter?

I believe that the best place to begin looking for information regarding this question is the July 5, 1942 entry in Anne Frank's diary. Anne's father tells her that they may need to go into hiding soon because the alternative is to risk being taken to a concentration camp by the Germans.
The following entry is July 8, 1942. Frank describes that a member of her family got "the call-up" from the SS. This means that deportation to a concentration camp is imminent for Frank and the rest of her family. They absolutely do not want to do that, so the family scrambles to get last-minute things ready to go into hiding. They plan to leave the next morning.
The July 9, 1942 entry tells readers that the original plan was to go into hiding on July 16, 1942; however, Margot's call-up accelerated the plan.