Ubisoft is pretty good about explaining its delays. This big interview on the Ubisoft Blog with Senior Producer Dominic Guay touches on general reasons behind Watch Dogs being pushed back, as well as specific features that came from it. The free roam multiplayer wasn't mentioned, but the multiplayer, invasion-esque singleplayer elements came up.
"We looked at it together with Ubisoft HQ. We thought we’d be done for Christmas. Everyone agreed. And that’s when we announced the date. And for quite a while, it really looked like we’d make it," Guay explained. "When we got close to the end, we still could have shipped."
The team was given leeway, however. "[Ubisoft HQ] said, 'We understand, we see the same thing as you, you guys need more time.' They agreed to it. I don’t know if I’m going to get that luxury every time I ship a game." That time allowed not only for polish, but for early ideas that fell by the wayside to make it into the game. They added the ability to send a high pitched frequency into enemy/NPC communication devices as a combat option, for example.
Why Was Watch Dogs Delayed? [UbiBlog]