Sometimes I see small capacitors, on the order of 1-2 µF in parallel to loads on DC circuits, like this:
What purpose do these serve?
Related but not a duplicate of Sprinkle switching circuits liberally with small capacitors?. That question is about how to use such capacitors when creating a circuit, while mine is about why.
The first misconception is that what you have in the picture is not a DC circuit.
It is an AC circuit that works at 1.5 MHz.
Sure it is a DC-DC converter but it switches the inductor to ground at 1.5 MHz to charge the inductor with current and then let it discharge the current into the output capacitor.
The capaitors are bypass caps. They store energy.
They have low inductance and resistance, and are placed right near to a circuit that needs fast energy pulses quickly. Or need to store fast energy pulses quickly. In this case, at 1.5 MHz switching speed of the chip.
The capacitor value, type and placement is chosen so that it can do the job well.
Larger, higher value, other capacitor type, with higher resistance and inductance further away with would be useless to provide large currents quickly.
The regulator repeatedly pulses current through L1, delivering energy to it, then stands off while that energy is transferred to C2. The regulator does this as often as necessary to keep C2 "topped up" to whatever output voltage is desired.
These transfers of energy from L1 to C2 are very small, but very frequent, anywhere from tens of thousands of "packets per second" to millions. Contrast this frequency with linear supplies, using bridge rectifiers; the capacitors you find in them have to be much larger, because they are required to maintain a reasonably steady voltage for much longer, "top-up" pulses from the rectifier happening only 100 or 120 times per second.
The input capacitor C1 in the switching supply you showed is to "smooth" voltage at the regulator's input, by acting as a short-term source of current as the load's demands change. However, because current is being drawn from that source in short, frequent pulses, current demand is very short-term, and only a small capacitance is required.
You find small capacitors across the power supplies of many elements in a circuit. There are usually long wires and PCB traces between a power supply and the elements being powered, and those paths have inductance which prevents current in them from changing rapidly. Therefore, any element that switches something rapidly on or off will find that the power supply simply can't keep up with changes in current demand. The result is dips and peaks in voltage at the supply rails, which is bad for two reasons:
Small capacitors across the supply near each element act as a short-term source of energy, able to respond to that element's fast-changing current demands. This helps mitigate the dips and peaks in supply voltage for the element in question and any nearby potential victims.
