Why and When should we use the capacitor array instead of switch or capacitor in the timer control?
I'm not entirely sure what you mean by "flip switch", but this circuit just seems to buffer SW_AD into SW_F with a variable delay set by Ct and Rt.

Basically, the transistors is just there to move the bias current on the left to charge the timing cap, and when the voltage on the cap reaches the voltage on Rt, SW_F is asserted. There is a configurable delay set by the capacitance, so maybe this sort of delay could be used for instance to configure the dead time or something like that. There is not enough information to really answer #1, other than the fact that this circuit adds a configurable amount of delay.

As for the second question, there is no good reason that it MUST be a 4 bit cap array. You can make it as wide as you want; but maybe the designer thought 4 bits is enough for the majority of use cases.