Explain the difference between the share of customers and customer equity. Why are these concepts important to marketers?

Share of customer is the share marketers get of the customer's purchasing in their produc categories.
For example my own consumption on coffee. 50% of the time I purchase coffee at starbucks, 10% on white coffee luwak, 10% on Kapal Api, and the rest on random coffee shops. This percentage always differs from one to another because it depends on the preference, and we understand that we people are all different in preferences.
To increase share of customers, firm can leverage relationships by offering greater variety to current customers. For example, Starbucks can add multiple new flavors to the coffee shop. Obviously, the advertising of the new flavors are going to be attractive to Starbucks customer.

Talking about customer equity, customer equity is the total lifetime values of your current and future customer.A company with high customer equity will be valued at higher price than a company with low customer equity.