A dead British science nerd figured out the best way to cut a cake to preserve maximum freshness, and it's pure genius (well, sorta). His name? Sir Francis Galton, a polymath infamously known as the founder of eugenics, and apparently owner of a very big sweet tooth.
Essentially, when you slice a cake the traditional way (i.e., in triangular wedges), you leave too much cake real estate exposed to air, which means it will dry out and get stale. Taking long, thin slices that cut right through the center of the cake will help preserve optimum moistness.
Realistically, though, how many people ever cut out a single slice of cake and then put it away? NO ONE. I repeat: NO ONE.
To be fair, Galton was British and lived during Queen Victoria's reign. High and low tea were meals that were taken seriously, and it wasn't uncommon for a single cake to be parceled out over days.
Just read any book by a Victorian-era author, whether that's Charles Dickens, or the Brontë sisters. Technically, Jane Austen is not a Victorian author, but her books are also chock-full of tea scenes. And let's not forget about current Victorian-friendly steampunk fiction, either.
So, unless you're eating a whole cake by yourself over a week's time, you might want to take Galton's cake-based findings with a grain of salt. Along with eugenics, he produced some other questionable-sounding studies like "Arithmetic by Smell," "On Making the Perfect Cup of Tea," and "A Beauty Map of Pretty Girls in Great Britain."
For us modern cake eaters, The Wire makes some other pretty compelling arguments as to why you can skip this whole cutting-cakes-via-science method. For instance, you need weird-sized "clean" rubber bands, possibly even a ruler, varying sizes of cake servers, etc.
NPR also points out that this cutting technique works only if your cake is frosted in fondant. If you love buttercream frosting, you're going to get the short end of the stick if you use this method.
If you're looking for more efficiency in cake-cutting rather than preservation, don't worry about slicing out the diameter—just do as you normally would, but use dental floss instead.