I don’t know much about ISO standards, but I have been familiarizing myself with EMC standards recently (e.g., electromagnetic emissions and immunity). These standards apply broadly to anything containing electronics and are designed to ensure that your hardware is not going to cause interference with any other electrical equipment (emissions standards) or be affected by RF noise, electrostatic discharges, etc. (immunity standards).
If you’re just posting designs online for people to build themselves, you probably don’t need to worry about such things; but if you ever want to sell a product, adhering to at least the emissions standards is mandatory in most jurisdictions (e.g., FCC in the US, Industry Canada, Europe requires compliance with both emission and immunity standards to attach a CE mark to any product, etc.).
Even in the simplest cases, emission testing can cost several thousand dollars (sometimes over ten thousand dollars for immunity testing). Selling non-compliant electronic widgets could subject you to significant fines and it may be difficult to ship things across borders without documentation of compliance. This creates a significant barrier to small-scale production of hardware, especially when you consider that any non-trivial change/revision you make to your product can require a retest.
There’s also the issue of safety standards (e.g., UL, CSA, TUV, etc.). From my understanding, these are not strictly required to legally sell a product, but they may be required by some customers (e.g., US government labs). Conformance with these standards requires some hefty fees and on-going testing (factory inspections, etc.).
I don’t claim to be an expert on these issues, but I am gaining a practical education while trying to commercialize DropBot, our wetlab microfluidics robot. I’m happy to share my experience going through the process if anyone is interested. Even if you don’t plan to “sell” a product, I would argue that it’s not a bad idea to at least know the basics and to try and design with them in mind.