Correct. You don't even need the wiring diagrams. Say you have to replace a switch... you simply unscrew the wires from the back using a screwdriver, remove the switch, slide a new one in and put the wires back where they were before. You really don't need an electrician to do this but if you're uncomfortable doing this then by all means get someone else. (Many of the people who build the panel themselves had zero experience in wiring or electrical and building it from scratch is considerably more work than replacing a single part).
You really only need the wiring diagrams if you want to build one from scratch and/or want to understand better how it works.
By using standardized parts you guarantee quick serviceability and that parts will always be available. Say, for example, a PID dies in 20 years. You can buy any similar PID from any manufacturer and drop it in as the functionality will be the same. There are hundreds of choices. It does not necessarily have to the same manufacturer at all (Auberins). The hole sizes are all standard understood manufacturing sizes (1/16 DIN for PIDs, 23mm for switches/lights, etc.) because the industry that uses these parts demands easy and quick serviceability. Pull the old one out, put the new one in.
I made a big investment in my brewing room and setup myself. I didn't want to be 'stuck' in 2, 5, 10, or even 20 years if something died and have to rework a bunch of stuff like trying to fit a smaller PID in a larger hole or vice versa. So the parts (relays, PIDs, switches, etc.) used in my control panel are all extremely common parts. They've existed for dozens of years and they will continue to exist because of the tens of thousands of (non-brewery related) industrial installations around the world running today that rely on them. I'm not married to one particular part or manufacturer. Not everyone would care for this level of serviceability or find using commonly available parts boring (not cutting edge) but it's something I wanted myself. I just want to make beer. I don't like to tinker with equipment. I'm sick of that - I just want it to work.
Kal