Buffer capacitors' role in boost power supply design?
That is done because the 100 uF capacitor isn't very "good" / doesn't work very well at higher frequencies. The 100 nF is much better at those frequencies, but has a smaller value so does not work for lower frequencies. The 100 uF capacitor takes care of those lower frequencies.

By using these two in parallel we create a "better" capacitor that works at both low and high frequencies.

It is explained in more detail by Dave from the EEVBlog in this video.