Answer:
Essentially, punctuation assists with clarity. It uses a variety of marks, each with a specific role, to create structure and organize language. It also assists when reading out loud by indicating pauses. What's more, punctuation can change the meaning of sentences. Sometimes this is overt; compare "let's eat, Grandma" with "let's eat Grandma". The first sentence is someone suggesting to Grandma that they eat, whereas the second is someone suggesting that they eat Grandma! On other occasions it can be more subtle:
"Alex didn't want to go the park today. We had been yesterday." - the full stop indicates they are two distinct assertions.
"Alex didn't want to go the park today, we had been yesterday." - the comma groups the two phrases suggesting they are related. That is, both phrases give reasons for why people don't want to go.
"Alex didn't want to go the park today: we had been yesterday." - the colon suggests the second phrase is the explanation for the first. That is, Alex didn't want to go the park because "we" went yesterday.