Blocked drains aren't just inconvenient, they can also be very expensive in terms of damaged property, and in commercial or industrial situations, can interrupt workflow or slow down production. When a drain becomes blocked, you need someone to get to it right away so things can quickly return to normal.
In times of drought, drain blockage becomes more common because underground pipes offer tree roots the hope of water. The roots will search for every drop of water they can get, so if there's even a slight crack in a pipe, tree roots will work their way in, and over time they'll block the pipe completely. Pipes can sometimes be cracked from the surface by vehicles driving over them or parking on top of them.