In stroke recovery what do leg braces do?

Many things. The braces are used to maintain alignment and prvent contractures. Also maintain stability when rehab is initiated.
Leg braces have seve. Leg braces help compensate for weakness that could be related to the stroke. Although the prescription and determination of need and specifics for bracing is best accomplished by a knowledgeable physician in conjunction with a physical therapist and an orthotist(someone who makes braces), bracing could help with walking. If there is foot drop better clearance of the foot is accomplished.