Why do some conjunctions use commas instead of periods?

If you pause with a comma before the conjunction, the tone is the emphasis on the content behind the conjunction.

Commas are added to pause and emphasize that conjunctions are function words, which can not be used as sentence components independently, but only serve as conjunctions and words, phrases and sentences.

Conjunctions can be mainly divided into four categories: coordinate conjunctions, turning conjunctions, selective conjunctions and causal conjunctions.