An effective argumentative essay must provide evidence because

evidence appeals to the reader's sense of logic.

the evidence will be used to support the counterclaim.

a conclusion cannot be made without it.

it explains the source of the claim.

An effective argumentative essay must provide evidence because:

-Evidence appeals to the reader's sense of logic

- The evidence will be used to support the counterclaim.

Even though argumentative essay tend to be fact based, it still often be biased based on the writer's own perception.