Before a shopper registers your colors or your UI, they register your type. Typeface, weight, and size communicate category and confidence faster than any sentence — which makes typography the highest-leverage design decision in your set.
Sans sells software, serif sells feeling
A bold geometric sans reads as product: capable, modern, efficient. A serif reads as editorial: considered, premium, human. Wellness and lifestyle apps borrow serifs to feel like magazines; productivity and finance stay sans to feel like tools. Neither is wrong — mismatched is wrong.
Scale beats decoration
The chart-toppers set headlines enormous — a third of the screen — in one weight, one color. Restraint at large sizes looks expensive; variety at small sizes looks noisy. If you want emphasis, change scale, not font.