We need both because clauses can be a complete sentence, where phrases are not complete sentences.
Now I have a question for you, why do we need both semicolons and colons?
We need both because they have different purposes, semicolons connect two complete sentences and colons list things.
What are dependent clauses?
A semicolon is used to connect 2 independent clauses together. Ex. It was super hot outside today; It was 95 degrees outside. A colon is used for listing items. Ex. You're missing everything: the pregame, the game, and the after party.
Dependent clauses are clauses that need the independent clause to be a complete sentence.
The subordinating conjunction connects a dependent clause to a independent clause or starts a dependent clause.
I wonder what the role of the subordinating conjunction...
Oh sorry, but I thought you were serious.
Excuse me, you are a very rude person.
Although you have only been answering my questions, today has been a fun day.