One of the first pieces of industrial machinery I got my new engineering hands on was a carton erector. It used pneumatics to grab flats, pull them into shape and push the carton out of the machine. It caught my eye because it looked like the OEM wanted to support the entire pneumatics industry single-handedly. I saw air-preparation hardware (filter and regulator) from one company, electric solenoid valves from a second, tube fittings from a third, vacuum generators from a fourth and pneumatic cylinders from three more. The memory of that machine and the component choices of its designer stay with me to this day.
Why did that designer put such a smorgasbord of components on that erector? Were they simply what was on the OEM's shelf at the time? Maybe that's what the distributors had available when the machine was constructed? Either or both reasons might explain it. In fairness, one 1/4-in-tube/90º 1/8-in.-NPT male fitting is pretty much like any other, so which one gets on a machine is probably not that important. And many manufacturers of disposable, stainless steel air cylinders make theirs dimensionally interchangeable with their competitors' cylinders. A two-position, single-solenoid valve functions like any other, so expediency might be one justification.
Cost is always a justification, sometimes too much so. Maybe all those different components represented the lowest cost for each class of part. Perhaps that OEM had a sharp purchasing department that constantly comparison-shopped for purchased parts. Each week the case erectors shipped might have different parts from those shipped the previous week because one manufacturer's valves increased in price, or another was having a sale on filter-regulators. The OEM can be sure of maximizing profit by minimizing cost with that method. Spare parts might be a challenge though. If the OEM uses its own part numbering system and has a good relational database, there shouldn't be a problem. That statement used too many conditionals; I wouldn't be happy with that design methodology.
Let's take a look at a different machine, one about which I know more. Let's examine a horizontal, form-fill-seal wrapping machine. Inside its electrical enclosure, we'll find contactors, servo drives, a variable-frequency drive (VFD), temperature PID controllers and a touchscreen. Each is supplied by a different manufacturer, just like the case erector. Another potluck case, it would seem. In this case though, I believe the choices were driven by a different criterion. Oh sure, cost to some extent played a role. But in this case, the OEM leaned toward picking best-in-class components (at least in the OEM's opinion). The components were picked for their performance and compactness. The VFD and servo drives in particular were impressive for what they could do for their small size. The components were the same for thousands of built machines, so there was no shopping for sale prices.
Best-in-class is not necessarily best-for-customer (at least according to the customer). The wrapping machine OEM had a problem with that best-in-class design: Its customers wanted components from a specific manufacturer. Many designers are faced with a customer-driven component specification: "Thou shall use what the customer demands."
On one hand, the machine now will have a consistent, single name on all the commercial components. It certainly will look impressive. On the other hand, the design might be compromised. Certainly for that wrapper OEM, the customer's specification required changing to a much larger enclosure because the drives were so much larger compared to the original compact drives.
On the gripping hand, the customer's specification means one-stop shopping for all the components. That might have a positive impact on cost. Hopefully the distributor will give a good discount for being sole-source to the OEM. It certainly helps simplify an end user's spare parts inventory if the equipment in the factory shares a common source. There's also the matter of training costs. The end user's maintenance department has to learn only one programming language, and there's less knowledge lost when a technician leaves. Expediency. Cost. Best-in-class. Single-source. If you're a lucky designer, all four criteria merge to a single solution. When they don't, then you have to be a good designer to bring these conflicting requirements into a customer-pleasing system.