Almost all new businesses are starting by having a tagline or at least stating their mission. Or if you want, making their initial sale pitch. Why is this so important?
I don't have any major in this field, so I'll just tell you what I think. Understanding the value of a new business is very complex and may take a lot of time. Your prospective target (clients, readers, etc.) are usually flooded by information and most probably they are lacking time. But you want their attention. At least for a moment.
People are usually allocating just a small fraction of their attention and memory to new things. Your goal is to take full benefit of this small slot. A word or a phrase or an image that resonates with your audience is your best bet.
You want to be able to catch their attention as quickly as possible. Explaining your business is not gonna work. At least not in that short time frame. So, you need to have a tagline, a mission statement, or a sales pitch: your 'catch-audience-attention-as-quick-as-possible' key phrase.
Let's see what experts (like Guy Kawasaki) have to say:
A vision that is clear, that is consistent [...], a vision that is unique (Gary Hoover)
You need to focus on your core competencies (Miguel Rabay)
Guy Kawasaki's "Don't Write a Mission Statement, Write a Mantra" is also addressing the subject: