If there is no agreed time then you don't know when the signals were sent and cannot make any sense out of the signal. They all have to send either in unison or according to a predetermined schedule. The synced clocks set that schedule.
The clocks are synced between satellites, but if your receiver is cold-booting in the middle of a forest, how does it know what time it is? It will receive a signal from 1 satellite that will say "I sent out this signal at time X", but you still don't know what time it is because you don't know how many nanoseconds it took for the signal to get to you, you can only be sure it's currently some time after X.
It will get another signal from another satellite, which could have the timestamp before X, because it left earlier and took longer to get to your receiver.
As I said, if you do the math, with 5 signals you can then determine your location in 3D space, and time!
