There’s lots of software out there that is available to use without payment, but is still license restricted in such a way that you are not permitted to redistribute, modify, use for commercial purposes, etc. To many, these rights are the far more important facet of “free” software, above what it costs.

But since the English language has the same word for all of these concepts, we have all these yucks running around with zero-cost but right-restricted software wearing the “FOSS” badge thinking they’re part of the club. So some people add “Libre” to the acronym to explicitly disambiguate.