Tracking the websites presences means that you can see the age and sex on the people that are mainly coming to your website. This mains you can aim the website more around that age group or sex. With this is mind you can also put in possible questionares in the check out process or after the purchase has been completed to see why/how they came to the website. Over time this would mean you could adjust the website further to suit this group.
A perfect example of Web analytics would be Google analytics, as you can monitor a wide area of aspects with Google. For example you can see where most of the traffic(users) is coming from so you can then direct more of your marketing towards those countries/places.
You can also see which are the most clicked on links on your website and also which is the worst links on your website. From this you can then edit your poorly attracted links to seem more appealing.
You can see if you need to adapt your website better for mobile users if there is alot more traffic coming from users that are searching via mobile. This will help you know where to put more time into when editing your site.
It is also free and easy to use, which is great for not only new users of analytics but older users too as it is a very good tool all round.