1)In this case the capacitor bypasses one of the two speakers for high frequencies. This is kind of a simple crossover network to account for the different power content of different frequencies so that the total of the acoustic output of all speakers sums up to a pleasant listening experience.
2) To protect the speaker from DC components in case of a failure within the amplifier. DC can quickly destroy the speaker. The capacitor for the subwoofer needs to be of a mucj higher capacitance than the ones you use for the tweeters.
That is a true 2-way crossover to separate the bass and lower midtones (speakers f5) from the upper midtones and treble (speakers sd1.1). Crossover frequency in this example is 4000 Hz, but that i not always the case.Bass speakers cannot reproduce treble and vice versa treble speakers (tweetes) cannot reproduce bass. By separating the frequency ranges the speakers are operated in their optimum frequency range for best sound quality.
Matching speakers, crossover frequencies and crossover circuit as well construction of the speaker housing is a science per se (
see e.g. here). It requires some math and lots of experience, not to mention the technical equipment (test tone generators, spectrum analyzers) and a good sense for sound quality to build a well soundig speaker.