Why is share of customers important to marketers?

"Share of customers" refers to the market share your product has captured. Example: let's say that Apple as 80% of the market share -- or "customer share" -- of digital music players with its iPod line, while Microsoft has 7% with Zune and all other companies split the remaining 13%. Apple has a huge advantage for future products due to their customer share -- when they release new iPod models or new iPod accessories, they already have a massive install base to market the new products to. It's far easier to sell things to people who are already your customers than it is to acquire new customers. Another example is internet search -- google has 80% of that market, and people tend to stay with what they have... it's difficult to take established "customer share" away for this reason. When someone releases a competing search product it needs to not just be perceived as "as good as google" -- which doesn't make people feel the need to switch -- but it needs to be perceived as "much better than google". Very difficult.