Theoretical physicists love symmetries and using very few assumptions to describe Nature. For example, people knew that there is "some force" that attracts an apple from the tree and there exists "some force" that binds the moon to the earth. But Newton realized they are the same force and their actions are described by the same simple law which is called law of gravity. This is the beauty that (theoretical) physicists are aspire to. They want to blend all the forces in Nature into One. That is, of course, an ambitious project.
The most fundamental step in this direction was taken by Maxwell when he unified electricity and magnetism together. That's not only beautiful but it gives us more understanding of Nature itself. We know now (after special relativity) that magnetic fields and electric fields are not fundamental objects but, rather, which one you perceive depends on your frame of reference.
The same logic works for any unification. It makes you understand the rules of the game (of Nature) much better than before.
About the electro-weak force, the unified theory is valid in high energy scales (around 100 GeV). Then it "breaks down" into Maxwell and Weak interaction as one goes to lower energy scale.
