If you're not reviewing your site's performance you don't know which pages are your most and least effective.
A simple content tweak might be all it takes to increase sales of a particular product.
Google Analytics is free, so you should be using it.
At a minimum you should know these things about your website:

• The number of visitors each month and the general trend (hopefully your site's popularity is growing month-to-month)
• The average time visitors spend on your site
• Which pages are most and least effective
• Where visitors are coming from when they visit your website
• Your top five keywords

Review some Google Analytic reports to know how visitors are interacting on each page and learn what percent of visitors purchase your product.
eBook X is selling like crazy, but eBook Y is not - and you really thought that Y would be the more popular of the two.
Find out why by taking a look at your page content.
Do you have more Buy Now or Add to Cart buttons on one page? Is your call to action stronger on one page? Are potential buyers abandoning their shopping cart? Google Analytics will give you information you can use to increase effectiveness.
Use your Google Analytics information along with your marketing plan to see what marketing campaigns work best.
For example:

• You send out your ezine on the 15th of every month and your web traffic always spikes on the 15th.
  
• You sent out a large email blast last month on the 21st announcing a new product launch.
  Traffic spiked only minimally compared to last month's product launch.
  
• Your blog typically is your largest referrer, but wasn't last month.
  (Maybe you missed one week of blogging).
  
• Last month's coupon offer increased conversions for product A by 5%, but this month, conversions only increased 1% using a different coupon promotion for the same product.
  

The amount of data you can analyze is staggering can be overwhelming.
Start by reviewing just the basics in the beginning and adding more sophisticated reporting later.
Also, Google Analytics gives you the ability to graph data for easy visualization, allowing you to view trends at a glance.
Set up a dashboard with just the information you want to see: top keywords, most popular pages, top referrers and more.
Compare this month to last month, or plug in various date ranges you want to compare to review trends.
Once you have an overall picture of how your site is performing, you can use that knowledge to more effectively market your products.
From conversions to product category performance, Google Analytics has you covered.