Yeah I think this is it really.The 'why' to me is clearly so that a bunch of different companies can pool their resources and share a spotlight instead of having to each individually market their own announcements. Without E3, a company like WB for example were probably like shit we were gonna do our own thing for the first time, now wtf do we do?
In comes Geoff Keighley with a basket for all these companies to put all their various eggs. This way people will be like ok that was a cool Inside Xbox... what's next? Oh ok some little indie thing... but what if SONY are after that!! Better stay tuned to this website.
Just anecdotally though, I think this is good for people like me as well. I totally missed the recent Stadia Connect because I don't go to places like IGN, I get my news from thread titles here and I managed to miss that thread. I don't really follow all the headlines for everything, normally, just games I care about. But every year at E3 I'll watch things like Devolvers show and the PC gaming show because they're part of E3. For me, having a centralised schedule saying what things are coming up that I'd miss if they were low traffic era threads will be really useful.