When a visitor lands on your website they have already decided on what they would like or expect to see. If they have clicked through from a paid for advert, then they would expect to be taken through to a page that is exactly what was described in the ad. If they have clicked through on to your home page then they will be wanting to be able to quickly and easily navigate through to the section they want to look at.
If your website is an e-commerce site then you need to ensure that the checkout process is secure but also easy to complete. Not all people want to have to set up an account to purchase an item or request a brochure / sample for example, so you may instead offer them the option to check out as a guest.
Website are always a balance between design, functionality and search engine optimisation. If you concentrate too much on just one of these areas you may find that your site simply doesn’t work as it needs to. Sometimes it is good to take a step back from your site and figure out what it is you actually want people to do when they land on it.