Suction pipes are simply tubes that transport fluids from one end to the other. That is, a pump system's suction line is a tube that transports fluids from a source to a pump. The simple definition of a suction pipe is that it creates a vacuum in order to draw liquid into itself.
Why in a centrifugal pump is the suction pipe of a larger diameter than the delivery pipe? This is because in the inlet side of the impeller we do not want the pressure to drop below the vapor pressure of the fluid which is being pumped. If it happens then cavitation will take place and pump will be damaged.