Personally, I feel like one of the best ways a sandbox could be used in an AVN would be - specifically in a harem/multiple LI game - allowing for a main story to be focused on, with "side stories" and/or side characters to be included (or even added later on, allowing for a focus on the core story before back-filling side content) without the worry of players needing to wait forever to see the main story progress while optional side stories get worked on (as so many VNs have happen) or even worse, expecting players to keep multiple saves or constantly restart because the dev goes back and adds things in to earlier parts of the game.
I've seen, on occasion, sandbox games where the main story is nearly completed before the dev then goes back and starts working on side content, essentially using the side content/characters the same way that other devs use a second game, as a way to stave off burnout/writer's block, which ends up letting them keep working on the same game, and more quickly jump back to the main story if they get inspired with what to add next, while working on the side content.
I think the biggest faux pas for sandbox, though, is trying to get too far into the weeds with time/dates for tasks, especially because devs seem to never have the ability to balance the tasks in a way that players don't end up with 10 different tasks that all need to be done at the same time, with absolutely nothing else to do for the rest of the day, so they spend days in a row doing nothing until that one specific time so they can do each individual task, instead of just being able to go through all of them in a tenth of the time. And even worse is when a sandbox game has the unavoidable events that force you into them, pass time, and then you lose your chance to do the task you were trying to do, and are only allowed to do at one specific time on one specific day of the week, meaning you have to go through an entire week just to try again.
The best sandbox games are the ones that just throw out time and days entirely, and simply let you choose to either continue with the main story, or do the side content, in my opinion.