 You already have a big 10 or 100 microfarad capacitor to smooth the output of your voltage regulator. Why bother adding an extra 0.1 microfarad cap? Simply put, using multiple capacitor values allows your board to filter out a wider range of voltage fluctuations. That big cap is like a tiny backup battery. When power is first connected, it'll fill up with charge. Then, if your circuit suddenly draws a lot of power, instead of stressing your regulator and causing voltage to dip, that big cap will fill the gap by draining its charge back into the circuit. Electrolytic caps like these are good for protecting against frequencies in the hundreds of hertz range. The smaller cap performs the same function, but for faster fluctuations, like say noise from outside interference. A ceramic cap like this is best used against frequencies in the 10,000s of hertz range.