To clearly answer your questions...
It is absolutely essential, vital, to have sales copy... and not just any sales copy... it must be done it a certain way, must contain certain components in a particular order to work properly. Sales is part science and part art, and both must be included in your page. Furthermore, sales copy needs to be on a landing page just for the product/service you are selling. Every product or service needs it's own sales landing page and should not have any distractions such as links, tabs, or a sidebar. Therefore, your "main" page isn't (usually) a sales page, per se.
To see how a sales page works, you need only to watch an infomercial on TV. They have the same components. A few of them are: The evoke emotion, they create urgency, they provide validation, they have a guarantee of some sort, etc... and they only sell one product for each infomercial
If you don't have the proper sales page, you won't convert well. I am not an expert with sales copy but I am an expert in business and I know the importance of sales copy!!