When to include "Previous" and "Next" button or link?

Happy new year guys.

I'm currently building a personal portfolio and I am thinking of including a "Next project", "Previous project" link where applicable to each individual project page. However, many portfolios I've come across only have a "Next Project" link but no "Previous Project" link. I think this, in a way, violates the symmetry principle. I also noticed for many websites there isn't even a "Next xxxxxx" link to begin with (like in e-commerce sites).

Can someone clarify when do we include a "Next xxxxxx", "Previous xxxxx" link to our page and why some people choose to only include "Next"?