If you do plan on becoming a Steam publisher, make sure you read the SDA carefully, because this is also addressed in detail (regarding release-parity on different platforms). Informally, it depends on what you are doing. I don't think there is a direct benefit to releasing on all platforms at the same time IF you are releasing on Steam, given that Steam is the big dog and will cover all your basis, both with regard to global accessibility, security and money conversion. If anything, it may fragment your user base a little if you release on too many. Ideally, you want as many sales on a singular platform as possible so that your game rates higher with regard to that platforms search algorithms.
The only real benefit to releasing on more platforms is getting the game in front of more eyes. Platforms like Gamejolt and Itch do both have better visibility than steam, at the compromise of having a smaller consumer base. With regard to Gamejolt specifically, I was talking with the creator (CROS) a few weeks back and he was talking about how he was thinking of changing the focus to be shifted more towards the development process of games rather than as a sales platform specifically (though the sales features would still exist). With that in mind, I think GameJolt is a fantastic platform when it comes to building exposure, though I haven't seen any crazy sales statistics within the platform itself, given most users have steam anyway, though there is probably no harm in cracking a version on a little bit after or before release, and it also supports them, but if anything, get a devlog going anyway prior to release. Our project has gained over 1000 new followers since we created our Gamejolt devlog, which will ultimately benefit our main release regardless of platform