For: Right off the bat, one page websites are ideal for mobile devices as visitors from these devices simply have to scroll to see what they need instead of having to tap and zoom etc. One page websites also keep all of your website content in one place, therefore your branding will be quite evident and arguably quite strong as it is delivered together all in one place. The sequence in which you want your visitors to interact with your content can be set on one page websites. As visitors will obviously have to scroll from the top of the bottom of the page; position what you want them to see first at the top and what you would deem a strong call-to-action or conversion positioned towards the bottom. In this way you can influence the user journey and user experience. One page websites also leave little room for error in that the page can be primed to only display your most important, cornerstone content with less need for filler text and images.
Against: One page websites can be difficult to search engine optimise properly. In theory, one single page anchors the entire website and may only be optimised for a single and general (and therefore most likely competitive) key word or term. With the entirety of a website, including all of its content, amassed onto a single page; loading times for this website can often drag and lag due to the sheer volume it has to run and load in one place. Likewise with all of the content in one place, navigational issues can arise if a visitor is looking for specific content that does not have a dedicated page to house it. This also presents restrictions when it comes to analytics. Instead of traditionally favoured methods such as being able to see the journey visitors take, one may have to rely on more singular methods such as heat maps to track interaction.
As you can see from above, there are a variety of arguments both for and against one page websites. As custom website designers, we usually prefer to flex our creative muscles across multi-page websites. But we will of course build and create one page websites (chock full of custom design and function features) should a client request it.