There are several reasons for the use or not of both buttons, many of them explained in different web pages easy to find with a simple Google Search (Scrolling vs. pagination).
I describe the reasons that seem most obvious to me from a practical point of view.
The interface design previously defined by desktop use is currently totally influenced by use on mobile devices. In fact, many decisions are made from the user experience on smart phones and then adapted to the desktop version.
If moving from one project to the next involves a screen change, the back button conflicts with the browser button itself, making it totally unnecessary because it's redundant and because it takes up space on a small screen:
If the change of projects is inside a carrousel, the
Back buttons are replaced by the slide forward
> and backward
As for the next button, the controversy is whether it is necessary, or is an infinite scrolling preferable as explained in this answer.
All this depends on how the portfolio design is planned and depending on the number of projects to be shown. Returning to the topic of device use, the user is more familiar with finger scrolling to view all content prior to clicking a forward or back button at the bottom of the screen.
- If the content is long, the navigation buttons may not be seen
- If the navigation buttons are floating they can cover part of the content, being a portfolio it's inconvenient
- In some portfolios it's better to show all the projects is a contact
sheet so the user clicks on the one they want to see in detail,
usually shown in a modal window.
- If the projects are subdivided by subject, there may be a submenu for
There are many examples, visit handsomefrank.com which is a wide gallery of illustrators.