Outdoor or whenever using available light, a reflector lets you use the same light for the fill-in. Using a flash you add a light that may not match exactly the main light, sometimes that's subtle visible.
Using larger reflectors, maybe on a windy day, can be more difficult - an assistant and/or stands and sandbags may be needed, usually the reflector needs to be close enough to the subject to achieve the required effect (and sometimes they can get in the way).
A flash, especially camera mounted ones (directly or using brackets) are simpler/faster to use, especially if the subject is allowed to move freely, or the fill effect needs to be quite powerful.
Indoor, unless you're trying to use ambient light, flashes and reflector light quality will be the same (unless toned surfaces of filters are used, of course), and it's mostly a matter of costs, space, desired effect, and other factors, i.e. it may be easier to put reflectors close to somebody's face than lamps.