Time is expensive. So, you can find parts elsewhere for much less, but it can take a while to find an equivalent option setup for prototyping. It's not as much of an issue if you've got a stack of blank SMD breakout boards. If you're doing some DIY work and you can find out someone's already designed a module that fits your needs, then why not.

Consider the DC motor/stepper feather for example. Perhaps it's $8 worth of components if you're sourcing things piece by piece. Paying a ~$10 premium when you're making a one-off design seems entirely reasonable compared to trying to design a board yourself, waiting for the PCB fab, etc.

I'm an EE and I source plenty from digikey/mouser/etc for personal projects. There's a gap for the occasional prebuilt board, break out board, or oddball electromechanical part (which can be a pain to find at other suppliers). Higher cost distributors like adafruit/sparkfun do have their place even when you have the background.

Hear hear. Buying a proven working board is a tool I have in my toolbelt that I can test against.

Have you considered they might be trading off on aspects that you don't care or don't know about?

As a sibling comment noted, a BOM might be $X compared to buying premade packages from Adafruit/Sparkfun at $X0. But paying $X0 means skipping over $X00 worth (or even $X,000) of schematic capture, board layout, design review, PCB fab, assembly, bringup, and fixing bugs.

I'm not sure why you're being downvoted. You're misguided, but have reasonable questions!

Here's the deal for me, cost-wise. I can go on Sparkfun, Adafruit, Digikey, Arrow, etc and get these known compatible pieces of essentially Lego that already have firmware support/libraries ready to go. Digikey (who stocks both Adafruit and Sparkfun parts) will have them on my doorstep tomorrow morning. Using the existing libraries, I'll have the a skeleton of a system up and running by tomorrow afternoon. I'll have the prototype up and running in 3 days.

Then, if everything seems to look like it's going to work well and I want to pursue the project further, I'll fire up Eagle or CircuitStudio and lay out a board. Upload it to Macrofab and wait 3 weeks for my fully assembled boards to arrive. And then port the firmware from my prototype over to the assembled boards. Or rewrite the firmware, depending on the quality of the specific sensor libraries.

It's all about bootstrapping the process. When I get a board from Adafruit or Sparkfun, I've got a pretty good belief that it's going to be high quality and fit well within their existing ecosystem of parts and libraries. Building a prototype out of those parts helps de-risk the project and accelerate the project by cutting out most of the up-front dicking around to get it working.

Plus, the vast majority of these boards also have schematics and part numbers provided. When I go to design a real board, I'll use those as references.

Making your own hardware only for tinkering it's pretty painful. I think it's better to make a functional prototype in a board like that, and then making your own board of you need to.

I was more saying that you buy the components and put the pieces together according to the datasheet.

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.

This is tantamount to saying that every degree-holding MechE should do all of their car maintenance and home improvement and design their own products. Genuine passion for engineering and a healthy dash of imposter syndrome impel me to handle as much of this as I can, but I don't hold it against others who don't.

Could it be curation? I'm still a novice myself so this comes from a place of genuine ignorance, but I find most of the channels for parts and whatnot to be overwhelmingly complex. Adafruit is dead simple to search.

You are an EE? You should know the major websites like digikey and mouser. They are really easy to search. They have tons of categories to narrow your search if you need something specific.

Quality control, time, etc. Just because you are a bricklayer doesn't mean you have to build your own fireplace.