Discoveryd was introduced in OS X Yosemite and replaced mDNSResponder -- both handling network management, specifically DNS services for OS X, iOS, and more. Apple has been trying to fix the poor Yosemite's Wi-Fi performance in almost every Yosemite release (10.10.1, 10.10.2, 10.10.3), but problems have still persisted; however, today's removal of discoveryd will hopefully alleviate many of the network issues users have been experiencing.
As outlined by ArsTechnica, discoveryd would cause a wide range of network issues including random DNS resolving errors, duplicate machine names (Apple TV 3, etc), inconsistent wake on demand, and very flaky bonjour registration.
Many believed the inclusion of discoveryd was built for AirDrop and Handoff/Continuity, but those features still work if you replaced discoveryd with mDNSResponder, and they still work in today's 10.10.4 beta. Currently, it remains unknown if Apple only temporary replaced the process to fix the bugs, and reinstate the service in the final build, but for now, we're enjoying is removal.
We'll keep you updated on future 10.10.4 betas, so be sure to follow iClarified on Twitter, Facebook, Google+, or RSS for updates.