why do we use colon

It is used for joining closely related independent clauses that cannot be joined with a conjunction.

I did not create these circumstances;these circumstances created me.

Here, the two clauses I did not create these circumstances and these circumstances created me are independent as they can stand by themselves as separate sentences. However, as they are related, it is better to include them as part of one sentence. Sentences can be combined either by using conjunctions (e.g., and, but, or), or by connecting them with a semicolon (as in the given case)

It is used before transitional words and phrases that join independent clauses.

I am not allowed to do this;however, in your case I am willing to make an exception.

Here, the clause in your case I am willing to make an exception follows logically from the clause I am not allowed to do this. Transitional words (therefore, for example, nevertheless, however, generally) are used for making the required connection between such clauses. As the clauses are related, it is better to include them as part of one sentence. Hence, a semicolon is used in such a case.

It is used for separating a series of items that contain internal commas.

The number of wickets taken by Joginder and the teams against which he did so are as follows: 39, South Africa; 35, England; 26, Australia; 24, Pakistan; 19, West Indies; 15, Sri Lanka; 15, New Zealand.