It's pretty easy to non-destructively lift a wax seal off of paper, with a sharp, hot knife and a bit of practice. You can then read the letter and reseal it with a touch more hot wax or the back of a heated spoon (to melt the back of the original seal).
Not that I've ever done that, as a courier, in a live-roleplay game, ever. Repeatedly ;D
Depending on the security level of the letter, of course, a non-letterlocked letter might be pretty readable even if sealed. A simple letter where the seal authenticates the sender but doesn't protect the contents might simply be folded in three and sealed closed - you can bend and flex such a letter without breaking the seal to read most of it. A more important letter being _protected_ by a seal might be folded into an ersatz envelope and then sealed on the join ... but that's most of the way to a basic letterlock.
So, like all communications there's a tradeoff between complexity and security, and whether you're using the seal merely to authenticate the sender (which was pretty common) or also to protect the contents.
To answer your question, the lifting technique I refer to works well on flat paper and variably well on non-flat paper. Something as delicate as a letterlock, I'd be seriously concerned about my hot knife nicking or tearing a part of the lock that I can't see under the seal ... and /that/ would be obvious.
So it's not so much that the seal holds the lock closed, as that the seal obscures the lock to the point that opening non-destructively is much harder.
It was a ~1000-person national level game in a fantasyish setting, and one of the game elements was that you were explorers and colonists in a strange new land, and you could send messages to your supporting factions "back home" to request support or supplies or whatever. Those messages had to be carried by a ship belonging to a trade house, and any supplies that your supporters sent you had to come back by the same method, and all the trade houses were represented by player groups on the field...
So yeah, a group of players would write to their "sponsors", and my group (as one of the trade houses) would open their letter, see what they were requesting and then forward the letter to the game organisers. The organisers would play the part of the remote sponsor, and send a letter back, which we'd open and read to see what orders the players were being given and what support they could expect (then we'd reseal the letter and deliver it). This positioned us super well to sell them exactly what they needed to achieve those goals, at an only slightly elevated "rush" price.
We didn't open every letter, and we didn't scam everyone - we made most of our money legitimately. But it was a fun side-game.
Not that I've ever done that, as a courier, in a live-roleplay game, ever. Repeatedly ;D
Depending on the security level of the letter, of course, a non-letterlocked letter might be pretty readable even if sealed. A simple letter where the seal authenticates the sender but doesn't protect the contents might simply be folded in three and sealed closed - you can bend and flex such a letter without breaking the seal to read most of it. A more important letter being _protected_ by a seal might be folded into an ersatz envelope and then sealed on the join ... but that's most of the way to a basic letterlock.
So, like all communications there's a tradeoff between complexity and security, and whether you're using the seal merely to authenticate the sender (which was pretty common) or also to protect the contents.