The English language needs phrases and clauses because without them we couldn't form coherent sentences. There would be words thrown around with out any meaning.
"I'll be here only half of today because of a doctor's appointment." "Even if you are, I'll still be here." "I know but I'll be back." "Before you go, let me day goodbye." "I will unless you don't tell me what I have to do for math and English."
The English language needs colons and semicolons because if we didn't have semicolons there would be very limited ways to relate sentences together. Also, if we didn't have colons it would be hard to have lists in text form. Again, it would be hard to relate it to other sentences.
Dependent clauses rely on independent clauses because dependent clauses either have a subject or verb and can't stand alone as a sentence. Independent clauses can stand alone as a sentence and the dependent clause adds more detail.
A subordinate conjunction connects and independent clause and a dependent clause. It relates them together.
Semicolons are used to replace coordinating conjunctions. Ex: The ice cream truck drove by; it was playing music. Colons are used to make a list after a complete sentence. Ex: The ice cream truck had many flavors: strawberry,vanilla, and chocolate.