I believe that's important for SOME juices, less important for others.
There are certain alcohol-based flavorings, and that could literally allow alcohol to evaporate out.
Or perhaps there some involvement with AIR that can assist the steeping process.
I will open a new juice, sniff, let some fresh air in, close it, shake it, and let it sit for a week.
I have found that basically unnecessary with Halo - they're big, they mass-produce, so you're not getting juice they made just for you. I presume occasionally, they'll ship to you from a fresh batch, and you might find it tastes different for a week as well...
Similarly, NicQuid comes presteeped - they're not big, but they do mass produce - it's a trade-off of (consistency and pre-steeped for limited selection)
I actually do NOT steep with the top off - but I have had "stinky" juices where I thought "if I left it open, some of the smell will HAVE to leave the bottle" LOL.
I've read it before but why cap off overnight? Is it essentially the same theory as with wine of letting it breathe?