Flat roofs, gutters and flashing joints are exactly where rigid sealants crack first, because the surface expands and contracts with temperature far more than a wall ever does. Liquid rubber gets around that by curing to a genuinely flexible, seamless membrane rather than a hard skin, so it moves with the surface instead of splitting away from it — brushed or rollered on, it follows every awkward joint and upstand a rigid sealant would struggle to cover properly.