Generally speaking, taglines aren't necessary. Luke Sullivan says, unless you can write a "Just Do It," just don't.

But here are two reasons portfolio students should practice taglines:

1. You're going to have clients who ask for them.
2. Taglines are just one more way to showcase how you think.

When I'm asked to write a tagline, I sometimes begin by asking myself, "What is this? And what does it do?"

The ultimate driving machine.
The uncola.
The king of beers.
The antidote for civilization.
Nothing runs like a Deere.
It gives you wings.
That was easy.
Think outside the bun.
It's everywhere you want to be.

Just a few examples of good tags that answer the simple questions, "What is this? And what does it do?"