The artifacts will work somewhat similarly to the legendary quest lines of the past 2 expansions where there's a leap forward in power each tier, this time in the form of a new relic slot. Also since the relics are what give the weapon its item level, it's needed anyway so that you can quickly jump up to the next tier of gear with a single relic instead of having to replace all of them right away.
Speaking of artifacts, it's worth mentioning again that the artifact traits (which apparently significantly change gameplay judging by the Aff Lock ones shown at Blizzcon) still are not in the build to be datamined.