The main purpose of a DCO is to take the portion of a CLA that guarantees the developer actually has a right to license the code under the project's license, as

The main purpose of a DCO is to take the portion of a CLA that guarantees the developer actually has a right to license the code under the project's license, as it's something that some users worry about using the software without. Basically so if someone copies some code from one of the Windows leaks into Linux and Microsoft comes to sue the Linux Foundation over it, they can point to it and say "we had legal sign off they had the right to license this code, please sue them instead". In terms of irrevocability, the DCO is not that different from most open source licenses who themselves claim to be perpetual. The arguments for exceptions to that being true all either apply to e.g. both the GPL and the DCO, or to neither. Generally the consensus is that an irrevocable license grant means irrevocable, though there are some concerns about examples set by e.g. the precedent that lets artists claw back rights from record labels after 30 years.