As traditional marketing venues become increasingly irrelevant, shoppers are using their own personal networks to decide whether or not to buy your product. Your customers are peer to potential buyers, so if you're not engaging your customers to make them ambassadors for your company, you're missing out on one of the most potent marketing strategies there is.
We're not going to discuss just technical issues on why people leave your site. Why not? Because it's easy and fast for a webmaster to identify bugs in the code and is (typically) easy to fix, but conversion "leaks" are harder to spot.