Waterproofing a planter box will protect it from developing cracks that would allow water to leak onto surrounding areas and penetrate internal areas if over, or adjacent to, interior finish work.  For planter boxes that reside near a buildings structure, waterproofing them will guard against deterioration of the structure thus preventing concrete cancer.  It will defend against water ingress onto balconies, decking, pathways and paved areas which would give rise to staining and excess moisture which promotes the growth of fungus and bacterial allergies, delaminates deck coatings and produces dry rot of wooden decking.  Additionally, the waterproofing of a planter box will provide it with a longer service life while also guarding against leaching of highly alkaline salts into the soil, potentially stunting the growth of plants.