This is a (very old fashioned) extreme example, but many systems are designed at least in part to hide information from the defenders. Bids on the left, explanations on the right:
1NT-2D; 12-14 Balanced - Artificial game force
2NT-3C; No 4 card major or 5 card minor - show your shape
3H-4S; exactly 3 spades, 2 hearts, 4=4 in the minors (key!) - to play
Sure the defenders know a lot about opener's hand - 13 HCP give or take 1 point, exact shape - but that hand is going down on the table after the opening lead, so they'd know that all anyway. All they know about the closed hand is that it has enough strength for game (which, well, they're in game, so one would hope so) and his decision on where to play relied on partner's exact shape (so, almost always 5 spades, but could be weak in hearts with 4 spades, and now he knows that 3NT has a fatal flaw).
As the other answers noted, all bidding leaks information, which is useful on defence (or in declarer play, from bids made and not made from the defenders). During the play, similarly, defenders will pass information about their hand to their partner with their carding signals, and that is read by declarer as well.
The key is that the information passed to partner is worth more than the cost of giving it to the opponents (usually). Another key, given in particular by 'Forget...', is that even with all that information, declarer play is hard, and defence is harder.
This ability to pass information with a very limited set of calls is so critical in bridge (even if it's read by the opponents as well) that the premiere strategy for the weak side is to take away as much of that room as safely possible, to make it that much harder for the strong side to work out where to play. This is called "Pre-empting", and entire books have been written about it, and entire nights at the bar after the game are spent discussing the "finer points".
As far as deliberate misbids, you are correct, they are legal but frowned on in many circumstances (against weaker players, when not in contention (especially against friends who are), just for the [...] of it,...). They also can not be done too frequently, ethically (as partner will get a better idea than the opponents when you're lying, and what you have when you lie. Since the opponents are entitled to all information you know about your partner's bidding, either you have to tell them (and then there are issues about whether that agreement is legal) or you don't (and you have an unfair advantage).) How frequent is "too frequent" is a question for the ages, but I do it more than most, and I probably average two a year (over 4000 deals or so).
It's not commonly done even ignoring those restrictions because passing information to partner is so important that lying to everybody actually harms your score on average (which is why "when not in contention", et al).
But basically, "full disclosure" is a base rule of bridge. Playing it with that constraint is no different from not being allowed to leave the batter's box to get a better look at the pitch, or not being allowed to play the puck after crossing the blue line before it. The best players are "the best players playing within the constraints of the rules", in any game or sport.