If it were just a money-saving measure, I'd imagine they'd have continued using Inter with whatever complementary monospaced font they had been using.
Vercel's motivation appears to be that, unlike other fonts, Geist was designed with "precision, clarity, and functionality at its core", which "enhances the visual experience of developers and designers", which in turn finally "empowers them to effectively communicate their ideas".
Vercel's motivation appears to be that, unlike other fonts, Geist was designed with "precision, clarity, and functionality at its core", which "enhances the visual experience of developers and designers", which in turn finally "empowers them to effectively communicate their ideas".