"I understand nets can be useful to some people for visualizing surface area, so it's important to teach about them, but I don't understand why students are REQUIRED to use them. ( http://www.corestandards.org/Math/Content/6/G/ , 6.G.A.4 )
If you're one of those people (like me and a lot of my students) who find it easier to picture each side & add up their surface areas WITHOUT mentally unfolding into a net, would you lose anything later by not learning about nets?" --@msjwright2
There are a lot of reasons besides Surface Area, but I'll just highlight some I see every year in class. Spatial Reasoning and Critical Thinking. Just because students are able to see one net of a solid doesn't mean they can see all relationships- Mark Chubb shared a good video on that this past week on twitter. Many students have problems with finding all the unique coverings of a cube: http://mathforum.org/dr.math/gifs/11nets_cube.gif
Students also get a better handle on geometric transformations from this type of exercise. Many of my students who have done this activity with me refer back to the "cube nets" when performing transformation on the Cartesian plane- it provides a great experience for students to draw upon for other math problems.
Just because a student can easily visualize nets doesn't mean that doing activities and building that experience for them isn't meaningful- it may be more meaningful that they initially realize.