Should I add certificates of authenticity to limited prints of my digital artworks?
The provenance that a certificate adds, when combined with a bill or contract of sale, is more than just proof that you did it. It is also proof that the holder has a legitimate claim to a legitimate artwork. (Kind of like the idea behind a bearer bond.) This can be useful for insurance claims or for reselling a work.

They can also be used as a means to show that there is a limit to a production run, such as 1/10. However, it is helpful to have this kind of document co-signed, such as by a gallerist or notary public.