After you have worked out (to your satisfaction) exactly what it is you want your website to accomplish, then how do you tell if it is working? Or, in other words, how will you measure success?
Let's suppose that what you really want from your website is to increase sales and find new sales leads.
If you have set a goal for your website to increase sales, then you must implement some method of tracking sales coming from the website.
If you have set a goal for your website to find new sales leads, then you must implement some method of knowing the source of sales leads.
Basically, you need analytics. If you know that you were getting 20 sales a day prior to adding the new product catalog and now you are getting 30 sales a day, then you can demonstrate an increase of 50%.
If you know that marketing was receiving 50 inquiries a week before the website and now they are receiving 150 inquiries a week, then you have 300% increase in new leads.
If you have no method of tracking sales or of counting new sales leads, then you have no method of determining changes in stats.
This seems very obvious. But until you sit down to actualize the process it may escape your notice that the current invoicing system doesn't have a data field for source. Or, you may not discover that it will become necessary to expand your reporting software to sort on sales source. And, it is not that impossible to imagine your sales people don't even collect information about where new leads are coming from. They could be so happy to have a new lead they don't even bother with "where did you hear about us."
All of the above you most likely are very familiar with, and should not need to be told. The part that may have escaped your attention is the importance of going through this process as early in the website design as possible. The reason is: your webmaster may need to revise the structure of the site to facilitate tracking of your goals.