In any electronic circuit, especially digital ones like this, current drawn from the power supply will vary, as things like LEDs and logic gates switch on and off. Sudden changes like these can cause momentary spikes or dips in the voltage provided by the supply, which propagate around the circuit.
The long wires usually used to distribute voltage (and carry current) to the various modules of the circuit, have inductance. The effect of inductance is to make it hard for electric current to change quickly. Think of it as like momentum/inertia of a moving object, which prevents the object's velocity from changing quickly. In a circuit which has things switching on and off quickly and frequently, causing sudden increases or decreases in current demand, that long wiring makes it difficult for the power supply and its distribution system to "keep up". If the power supply is suddenly asked to provide an extra 10mA for an LED for example, inductance makes it impossible for the supply to instantly comply. The consequence is a momentary drop in supply voltage, while the power supply and its wiring "catches up".
There are usually several elements connected to that same supply, and they will all "see" these fluctuations, and can suffer to some degree. For example, such a supply voltage disturbance might cause an audible "pop" in an audio system. In your circuit here, you have a very complex microcontroller and display system, in which a moderate supply disturbance could flip a memory bit or even cause a reset, or any number of glitches.
The presence of capacitance between power supply rails acts as a short-term reservoir of energy, able to supply extra current when demand spikes suddenly (and temporarily), or absorb excess energy when demand drops suddenly. This has the effect of "smoothing out" the potential difference between the two supply rails, keeping that voltage much steadier than it would be without the capacitors.
Capacitor C10, to the left of the 7805 regulator in your circuit, performs this function. That regulator is very good at preventing voltage disturbances at its input from reaching its output, but not that good. C10 lends a helping hand to the regulator, so that severe fluctuations in voltage (from an essentially untrusted 12V source) are quenched to some degree prior to regulation by the 7805.
On the output side of the regulator, you also have capacitor C9, which performs a similar function, on behalf of the the circuitry following the regulator. But C9 also has a second purpose. The 7805 (especially older models) actually requires some capacitance across its output in order to be stable. Without at least a few microfarads there, it's possible for the regulator output to oscillate, which can be devastating to any sensitive circuitry connected to it. All regulator datasheets tell you the minimum (and sometimes maximum) capacitance that can/should be present across its output.
Fat electrolytic capacitors, like those 470μF devices C9 and C10, are good at accommodating moderately fast changes in current demand, but often you will also find much smaller (1nF to 100nF) capacitors across the supply too. These would be present to mitigate really fast spikes and dips.
The actual values and construction of these capacitors would be chosen according to the conditions they are expected to operate under. 470μF seems excessive to me, for such a small system, but it could be that the designers are anticipating that you will be adding additional loads, perhaps badly behaved ones like relays or motors, later on. Typical values to find immediately either side of a regulator such as the 7805 would range from 10μF to 220μF, depending on the expected current through the regulator, and the nature of the loads it is supplying. Again, the datasheet usually provides recommendations for choosing capacitor values.
If we don't know what the source of 12V is (at the regulator's input), a large capacitance there (well above 220μ) is not uncommon, since we have no idea of how clean that supply is. However, if you knew that the 12V supply was very clean, and very stable, you could probably reduce C10 to 10μF. Lower value capacitors are physically smaller and less expensive, which might be important to you.
It's difficult to say how big these capacitors should be, because every application and regulator has its quirks and idiosyncrasies, but values you choose for C9 and C10 here would range from tens to hundreds of microfarads, for current demands ranging from tens to hundreds of milliamps. That's a really "ballpark" guideline though.