Why do you need a patch for both eyes after retinal detachment surgery?

Usually not:: After retinal detachment surgery, some technique is done to close the tear or opening in the retina. If gas is used, one needs to be quiet so that the laser or freezing technique used to cause an adhesion around the tear so it seals, one needs to be quiet and still enough to allow it to heal. Some patch both eyes to keep the person quiet for a few days.

Patch after surgery: The patch is used as a precaution to protect your eyes from accidental rubbing. A few sutures are placed at the end of the surgery and your doctor does not want them to come undone at this time. The patch also helps to minimize infection...

Just on one eye: The eye patch is protective for the operative eye only. It is not common nowadays to have both eyes patched if only one eye is operated on.

Limits eye movement: Your doctor may have patched both eyes so that you don't move your eyes. Depending on the situation, your doctor may have bilaterally patched you because both eyes move when you try to only move one eye. This type of patching is very uncommon nowadays.