Book value per share will help people who own the share determine the value of their shares if the company should close and liquidate their assets and paying anyone they owe money too. This is important because if you invest in companies that are not stable and may be speculative, you want to know how much you will potentially receive should the company go under. Obviously one should not invest much into these types of situations, but there are times and places where this could be beneficial.
Book value per share could also be a way to measure the potential risk. If you determine a book value to be $10 and the stock is trading above the book value, you could use that as a stop loss. Measuring risk is important to any portfolio and should be done carefully because profits are good, but being safe from risk is also just as good.
Book value will likely change because outstanding share may fluctuate as stock is either pumped into the market or taken out. Also, be sure to look at the top of the equation and understand what is fueling these numbers and how it might give you an edge in investing. If you get stuck, there are wonderful tools on the Internet that can help show you how people use this in their research. Join an investing community and see how others are using this tool in their research. This will allow you to see real time examples and as real time questions. Lastly, if you need more guidance, reach out to an investing professional and they can help show you the best ways to use this tool and implement it in your current investing research.