Selecting components according to data sheets, capturing the schematic, designing the PCB, designing mechanical components/enclosures, ordering components, orde

Selecting components according to data sheets, capturing the schematic, designing the PCB, designing mechanical components/enclosures, ordering components, ordering bare PCBs, assembling PCBs, bringing up the PCB takes time and is not foolproof. If you're looking to prototype or to make something in small quantities for your own use, it is much faster and less error prone to order something like this.