Most entrepreneurs don't create mission statements for their companies.  This happens for a few reasons.  First, most mission statements that serve as examples to entrepreneurs are done poorly making them ineffective and giving this exercise a bad rap.  As a result, I find that lots of entrepreneurs choose not to create them, electing not to waste time on what appears to be a lame management tactic.  Second, many entrepreneurs never think about creating them, especially when the management team is hyper focused on creating product, business plan or raising money. 

In my opinion not creating one is a huge mistake.  Developing a clear mission early in the company's development is critical.  There are a number of reasons for this.

The first set of benefits are tied to the process of creating the mission statement.

First, the process of creating a mission statement is hugely beneficial to founders.  Since it's typically difficult to write less than more, boiling your company's over-arching goal down to one sentence is a daunting and nuanced task and the exercise of doing so should help the founders get on the same page.

Second, the process of defining a mission will help managers determine whether or not the scope of their effort is too big (and unnecessarily competes with incumbents) or is too small (and doesn't capture the real market opportunity).

Third, if you find that your mission statement is to create a better version of something that's already being done, your company may not be differentiated enough to make it through the throws of being the small player going head to head with incumbents.

The process of creating a mission statement defines and clarifies your team's objective, giving your team a polestar around which they can build their entire strategy.