Reading the description of manacles, they don't seem to actually do anything rules-wise mechanically when used. Am I missing something or am I just being too literal?
Do the manacles actually mechanically impact the creature that they're on?
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In the Tomb of Annihilation adventure (p. 119), manacles and shackles have the additional effects:
The manacles impose disadvantage on ability checks made to climb or swim. The shackles reduce a prisoner's walking speed by half and impose disadvantage on ability checks made to jump.
In the Storm King's Thunder adventure (p. 177, 181), manacles reduce speed by 20 feet:
The dwarves have ankle manacles that reduce their speed from 25 feet to 5 feet.
The non-ore prisoners wear manacles on their ankles that reduce their speed by 20 feet.
Manacles are defined in the rules as follows:
These metal restraints can bind a Small or Medium creature. Escaping the manacles requires a successful DC 20 Dexterity check. Breaking them requires a successful DC 20 Strength check. Each set of manacles comes with one key. Without the key, a creature proficient with thieves' tools can pick the manacles' lock with a successful DC 15 Dexterity check. Manacles have 15 hit points.
Manacles literally "bind a creature", what this mechanically causes to happen would be entirely up to a GM.
If you wanted manacles that have more of an effect you would need the Dimensional Shackles which state:
You can use an action to place these shackles on an incapacitated creature. The shackles adjust to fit a creature of Small to Large size. In addition to serving as mundane manacles, the shackles prevent a creature bound by them from using any method of extradimensional movement, including teleportation or travel to a different plane of existence. They don't prevent the creature from passing through an interdimensional portal.
There are several other items which similarly have no mechanical benefits (or even less than manacles) such as: a Mess Kit, a Chain, Grappling Hook, Hammer, Sledgehammer, Miner's Pick, Piton, Iron Spikes, and a Whetstone. All of these either lack descriptions entirely or their descriptions do not mention any mechanical effect the item has (such is the case with the Chain and the Mess Kit)
There is a problem that can easily happen when trying to define the use of an item, especially the general use items like manacles, or rope:
whether made of hemp or silk, has 2 hit points and can be burst with a DC 17 Strength check.
What you don't want to happen, as the designer of the text, is for the description to be so specific that it allows for strange and weird behavior that is opposed to common sense of how the object is being used in any particular occurrence.
For instance, if it was declared that the object gives any specific condition, or prohibits specific kinds of actions, manacles could be attached with each end to two separate creatures and both of these creatures would have this condition, even though that's not really how manacles actually work.
You could continue to add to the mechanics, specifying how manacles or rope can be used and all the different effects that each different usage would have on the creatures involved, but doing so is either going to be an entire book by itself, "I'd like to see them take into account creatures with more limbs than four, or random usages like using them to zip line", or there's going to be limiting number of options, which is entirely against the nature of d&d.
Instead, the game does not attempt to describe what specific effects happen from the "binding" that manacles have, letting the DM interpret how they might be binding things together and what that means for the situation, and instead focuses on what can be done to the manacles and the DC of commonly-attempted actions against them to end the effects.
Here is another example from official text if you are looking for inspiration. In Out of the Abyss there is the following description (pg 4, emphasis mine):
... wear iron slave collars along with manacles connected to iron belts by a short length of chain. This leaves the prisoners restrained, but doesn't effect their movement or their speed.
None of the conditions fit having wrists, ankles, or both bound by manacles.
It seems clear* that being bound by manacles is intended as a source of frequent Disadvantage, at the DM's discretion.
*First because things would not be included in the rules if they could not affect the game. Second, common sense.