Why do PCB circuits have resistors and capacitors connected to ground？

Ground is a pretty common place to connect things because
(a) it is the voltage reference point, and doesn’t move around
(b) it is the lowest noise signal and
(c) it is the lowest impedance connection available.
It is also a good idea to remember that ground is a rather complicated subject. A smartphone has a ground plane inside, but it isn’t connected to ground. What is it? Many devices have multiple “grounds” for different purposes. Often they are actually connected together at only one point. Why are there both analog and digital grounds? Why is the “neutral wire” in power wiring connected to ground only at the circuit breaker panel? Why is the shield in microphone cables connected to ground only at one end?

For example, suppose you see a schematic, with, say, an operational amplifier, and you notice a 0.001 microFarad capacitor connected between the Vcc pin and ground.

This component is there
(a) to provide energy storage for sudden power demands by the op amp, and
(b) to reduce noise and interference which might enter the op amp from the power supply.

Obviously one end of it must be connected to Vcc, but what about the other end? For energy storage, it could connect to ground, or it could connect to Vss, in some sense it doesn’t make any difference, but ground is usually the lowest impedance connection point, and that is what matters. The same arguments apply to the usage of the capacitor for noise reduction.

In a common emitter amplifier, configuration, you'll have a voltage divider network on the base, with one resistor connected to the power supply, the other end to the base, another connected from the base to ground. This forms a bias Ing network to set the forward bias on the transistor. On the emitter, you’ll have a resistor connected from the emitter to ground, to set up the other voltage necessary for forward bias throughout the entire cycle of the input signal, so voltage on the emitter never reaches zero. In parallel with that resistor, is a capacitor, one end connected to the emitter, the other to ground. It serves the role of a bypass capacitor, to shunt the AC variations on the emitter voltage, which would otherwise be present, and cause a sort of negative feedback. In other words, using an NPN transistor, as the input signal went positive, current flow through the emitter resistor would increase, increasing the voltage on the emitter in direct proportion to the input signal. Since the increase in current (and amplification) of the transistor is dependent on the increase in the difference of potential between the base and emitter, these fluctuations on the emitter would reduce gain. The capacitor in parallel with the emitter resistor keeps the emitter voltage (relatively) constant regardless of the variations in current flow caused through the device by the input signal.

RB= Base Bias Resistor—-provides forward bias on the base emitter junction

RS = Base Stabikization Resistor——provides a voltage divider with RS to keep DC bias constant

RE= Emitter Resistor——prevents Emitter from reaching 0 volts during negative cycle on input

CC= Coupling capacitor——-provides DC isolation between stages

CB= Bypass capacitor——shunts AC signal on Emitter to ground, keeping bias isolated from input caused changes.

RL= Load Resistor——Develops load voltage