WHY should I put comma after a dependent clause?

For example,

If you vote, you can have a say.

Or you'd also have to put a comma after any other dependent clause beginning a sentence. So we should put a comma after dependent clause—why?

It is not required to put a comma after a dependent clause, and some writers don't. Here are two examples from journalists writing in today's Guardian newspaper:

Unless the public gets angry enough to force a rethink we had better hope that at least the computer stays risk-averse.

Every time a "periodic" falls off the wagon they hit the ground harder.

Nevertheless, except in the case of very short dependent clauses, it is generally courteous to the reader to insert a comma, since it makes the sentence easier to parse. In some cases omitting the comma will result in a momentary ambiguity:

While I was cooking my daughter did her homework.

If you do so, it will be because it marks a prosodic break in the sentence: If you vote and you can have a say are separate breath groups. There may or may not be an audible pause between them, but there will almost certainly not be a pause elsewhere and no pause between vote and you.