Do you really need to use the spigot to center the tool?
Probably not, but the spigot provides an absolute reference point and a higher degree of accuracy. In the worst-case situation you have two or more holes poorly hand drilled and the centering on them is off, the accurate spigot will be necessary. But you're right, in most cases you can center on what studs are there. In fact, on my first homemade drilling jig the stud holes were oversized so taht there was enough play in the jig to accurately index it to the stud being repaired before it was securely bolted down. No spigot adapter and yiu could only do one stud at a time, btu the price was right, even 30 years ago.