Marian Said:

What are some examples why teachers should differentiate their teaching style and lesson plans?

We Answered:

Students don't learn in the same way.
Some are kinesthetic, some are auditory, and some are visual. Some have learning disabilities - mental or physical.
Some need a combination of kinesthetic or visual learning approaches before they can fully understand something.

Outside of that - attention spans and interest also play a role. Even if the material can easily be grasped, they might not be interested. The teacher has to play the role of entertainer to keep attention spans.

The reasons are numerous.