Conjunctionen create a logical or a timly connection between sentences, phrases or elements of sentences.
Connection between main and subordinate clause
I tell you this that you understand.
I tell you this, because only you understand it.
He is not only arrogant, but also boring.
Connection between elements of a phrase
He either eats potatoes or rice.
He eats potatoes and rice.
He can neither read nor write.
Connection between two main clauses
He buys a car, and she buys a motorcycle.
There are always efforts to sort things, also conjunctions. Since we do not see a certain advantage in sorting them by their function, we do it by meaning. The only important thing to remember in connection with conjunctions is, to know when to use the indicatif or when to use the subjonctif.