What is the purpose of changing strings on the acoustic guitar?

Because strings degrade quite quickly even if the guitar isn't actually played. Just being under tension and exposed to the air has an effect on them. Obviously if the guitar is played then friction wears the strings out and the windings fill up with dead skin cells and sweat.

Over time wear might mean that the strings don't play in tune, they will sound dead, feel unpleasant and eventually break.

There's more than one purpose.

Sometimes, one may want to change (all) strings to change the timbre of the string. Nylon strings sound different than metal strings.

Sometimes, the strings just need too be replaced after many months or years of being under tension just because they were tuned and the guitar ready to be used. That tension causes the strings to stretch and are more prone to breakage. Replace the strings before they break.

a] they sometimes break

b] they lose their brightness and sustain.

Lots of reasons-- a string breaks (meaning others are soon to follow), strings get stretched out and it gets hard to tune and doesn't stay in tune, you want to try out some strings somebody gave you or recommended as being really good, strings have gotten corroded because it's been a while since you last played it, switching type (like steel to nylon) to get a different sound...