System fonts are the absolute bottom of the barrel. Some are well designed but using any of them is a visual shorthand that you didn't care enough to put thought into your design. You're associating your product with the ocean of amateur work on the internet, giving the impression you copy pasted a template.
There are some high quality free fonts typically backed by massive organizations with actual typographic expertise. Most free fonts however, are amateur work that are technically and functionally lacking. Professional fonts are well designed at all weights, they're carefully spaced, they include much larger character sets to support more languages, contain features like lining and non-lining figures, variable font weights, small caps... are those all slight differences?
There’s a reason so many articles exist with titles like “Google Fonts That Don’t Suck”. Most of them do. If you are a professional whose job requires working with type, then choosing a font is foundational to your product. Arguing that all design is BS is just lazy; it's not a coherent argument.
I highly recommend practicaltypography.com, a free web book that discusses all of this and more, including why system fonts are bad and why a professional typeface is worth paying for.