A well designed and correctly distributed flyer can reap huge rewards.
Using offers and response mechanisms, such as voucher codes, can be great targeted marketing with a means of measuring your ROI.
Leaflets can succeed where social media advertising doesn't, because they can directly reach a potential consumer. A potential client taking a leaflet from you at an exhibition, or picking one up from their favourite store, is more likely to be engaged with it's content. A leaflet also benefits from its tactile nature and has a 'shelf-life' well beyond a social media ad impression.