It's not the idea of advantage to the sealing player 'cause he knows the move that will resume play. That's not an advantage to him anyway, and there's a far, far more important issue that arises if he doesn't seal a move.
If he does not seal a move, and it is, oh, 17 hours until play resumes, he gains 17 hours... 1020 minutes of time to consider his next move and be ready to play it. So, if each player ended up using two hours of on the board time, the sealing player had 19 hours to consider his moves while the other player had 2 hours to consider his moves.
To kill off the niggling quibble here and to point out why it was never much of a "knowing the next move advantage" consider the ways you can look at it. I am referring only to time used to consider real, my move is next, what shall I play, oh, and I know everything there is to know about the opponent's moves since they've all been made, kinds of moves. Yeah, sure, I can think "on my opponent's time" but he could play lots of moves and if he's thinking about things while I think too, then clearly at least two of them intrigue him. What are those two? Maybe that's easy to predict, maybe it's not. I have quandaries and some amount, maybe a lot of it, of the time I think "on my opponent's time" is pointless as the opponent makes some other move altogether, or since he can only make one move, the time I spent considering his other options was rendered "wasted."
If I seal a move, then BOTH of us analyze from that point with one opponent's move "in between" the position now and our next move. In his case, my unknown sealed move is in between now and his next move. He must waste some time pondering paths forward that won't happen due to my sealing a move that makes other paths happen. But... I may know the next move that will happen, but the move he must make, the one I just mentioned, is in between the move I sealed and whatever next move he makes. Both of us have plenty of time to think about everything but both of us have one opponent's move in between now and our next move and that opponent's move could be many things.
Sealing is and was simply a method to maintain that fact and apply fairness to all.
Remember, if I don't seal a move, there is no opponent's move in between now and my next move. EVERYTHING I spend time on is then worthwhile, nothing wasted, for me. A huge advantage over my opponent.
I'd also point out that after whatever time control after which adjournment was allowed, EITHER player could choose to seal his move. Not just, say, White. That had a minimum of two important effects.
First, why would either player seal a move in a situation in which his next move was dead obvious (due, for example, to hanging a queen if he doesn't seal a particular move... might as well be clear about how advantage could be had there)? Do so, and you might are adjourning the game in a situation in which the opponent is essentially thinking "OK, if he doesn't make "the move" I take the queen and win. That was four seconds. So, let's spend the other 16:59:56 doing real analysis assuming he makes "the move." Yay, this is soooo sweet!" So almost by definition, a sealed move will be one in which the position has some amount of complexity.
Second, a player could actually choose to aim for "very complicated" so as to minimize his opponent's benefit from the extra thinking time. The opponent would have to consider all the reasonable moves while he knows the move chosen and might enter his analysis looking at a much simpler future, thereby gaining an advantage. I'd say two mitigating things though about that advantage. One is that most complex situations seem to arise over a few moves, not instantly after a single move, and as a practical issue, why would you want to give him the, say, 17 hours, to think about the "sitch" instead of making him burn clock time? So moderately complex situations, not clock burners per se, but not 2) ... Kc6 either, would seem the target and most folks playing at a level in which sealing occurs ought to be able to handle things without the opponent gaining large advantage. The other is that either player can adjourn, so your opponent sees you're aiming at something and adjourns a move before you intended to. Now YOU have any disadvantage that arises.
So basically, it is/was just meant to make adjournment not the source of possibly incredible advantage to one player. Everything else is details and small plus/minus effects. EACH player is in the position of analyzing all he cares to, but always while not knowing his opponent's next move. Not just the non-sealer. Just keeping the playing field as level as the situation can permit.