1) We communicate using sounds - not movements of the tongue
or other articulators. Articulatory movements are merely a means to
an end. In many ways, it doesn't matter very much how the sounds are
generated: whether by a human vocal tract, a speech synthesis program on
a computer, a sound recording on CD, or on the TV, or a parrot.
(Jakobson: "We speak in order to be heard")
2) We can actually produce a given sound in various different ways. For example, interdental vs. postdental [θ]; [a] with clenched teeth. Contrarily, a given movement may yield different acoustic effects (e.g. [ti], [ta], [tu]). Therefore, we want to understand the way in which sounds are generated in the vocal tract.
3) Many movements go on together in articulation, but we hear a single stream of sound.
4) Meaningful differences between words are encoded as sound differences. Some natural classes (e.g. labials and velars; retroflexion, rounding, pharyngealization, dentalization) make sense only in acoustic terms. Some aspects of speech (e.g. friction) can only be properly defined in acoustic terms.
What is sound?
How are sounds are generated in the vocal tract?
How do sounds differ from one another?