Why would you want to a release about your game at same time with all other games, if you didn't have to. You would instead try to guess a few days in year where there isn't likely other releases and aim for that. In later case you get more publicity for same effort.
E3, kinda makes sense for connections and getting boots on grounds and showing stuff in progress in controlled environment. But for online you are better off going alone, unless you are tiny.
This is more about back in its peak, but E3 used to gather all the gamer's attention to a single keynote, it was like the Super Bowl of gaming. Basically if your game was on stage/shown off well it would be covered by every single gaming press and magazine and almost guaranteed mainstream gaming culture hype.
Today Geoff Keighley's Video Game Awards basically took over that role and similarly is the single biggest stage/audience to announce a new game to. If you had a game that they were interested in revealing + you thought it would sparkle to its audience it's a powerful platform.
E3, kinda makes sense for connections and getting boots on grounds and showing stuff in progress in controlled environment. But for online you are better off going alone, unless you are tiny.