I can speak to this a bit since we've been hit with leaks before.
Frankly, it sucks. A LOT. At Moon, for an event like E3, the whole team usually starts to put their heads together as to how we'll surprise players around NOVEMBER already - and then the work starts to put a trailer together. We first do quick animatics, often record gameplay of unfinished sequences or zones that need to end up in a finished state in the trailer and we then labor over every single frame for the next 6 or so months. Everyone in the team is affected by this, our design team needs to finish all the scenes that will be shown, our artists need to make sure every single texture and particle is perfect, our animators need to polish up animations, the FX Department needs to finish all the effects, our Tech department needs to make sure all the tools and everything works perfectly, usually Cassie (our editor) is making tons of tons of new cuts just to make sure that the music and the footage fits perfectly together... it's a crazy shit ton of work where you quite literally labor over every single frame and watch the cut hundreds of times over a 6 month period. And we do that to give fans something that makes them happy, to surprise them, to re-connect with our fans, so that they finally get new information. To me, E3 is all about fun and surprises and just go to YouTube to see the reactions of people watching the Ori trailers for the first time. Whether you find it silly or not, we had lots of fans bursting out in tears once they saw that we're making a new Ori game simply because they got so excited over the announcement. THAT's why we put so much work into this stuff.
Imagine you're planning a huge surprise party for someone for over half a year - that's basically what E3 is for us. We have to keep everything super secret during development, so E3 is one of the few times in the year where we can reach out to our fans, give them more information and just re-connect. And now imagine someone comes in and spoils the whole surprise party and on top of that, you run the risk that the whole party gets called off, even though you spent half a year working incredibly hard on it -> That's why developers hate leaks. It's hugely demotivating to have someone just shit on everything you did, to potentially lose a huge marketing asset that you've been laboring over, it potentially hurts the game and the studio financially and all that just so some journalist can boast that they've got cool insider information. I completely get that it's a journalists job to pick up cool information that they can then share with the public, but just think for a second how developers are affected by this. We had screenshots of the initial Will of the Wisps trailer go up before the announcement and I saw how everyone in the team just got depressed when they were all super excited just days before to finally be able to show off their work. And at that point we were unclear whether Microsoft would pull us off the stage completely because of the leak - so yeah, it's just a huge gut punch, to be frank.