An Orthodontic Device

Retainers are something that is required after the braces treatment is complete.
This is not necessary as it depends on the patient's needs.
If the patient don't need it, then retainers won't be needed.
If a patient needs a retainer but don't wear it, his teeth will relapse, or return to normal. So as you can see, retainers are important too.
Basically a patient can wear the retainer at night but if the orthodontist should tell you to wear it at day too, then do so.