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It's pretty hard to mention ALL the reasons, but here are a few:
1. Ripple voltage can feed through the power supply, due to its finite ripple rejection
2. A switching power supply will generate ripple at the switching frequency, due to parasitic elements in the output capacitors (ESR, ESL) and due to their finite capacitance.
3. Voltage drops across connecting traces, due to currents varying in the circuit and traces having non-zero impedance.
4. Variations due to changes in the circuit elements (resistors, reference voltage, etc.) due to environmental factors (temperature, humidity).
Thus, the power supply rail will have some noise and ripple on it, which your circuit should reject. The parameter that characterizes this ability is the PSRR.
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