QR codes are bridges from physical to digital with attribution attached: the truck door to your reviews page, the invoice to the review ask, the yard sign to the landing page, the trade-show banner to your calendar. Post-2020, everyone's camera knows what to do with them.
The two rules that separate working codes from decoration: every code carries UTM tags (so the truck and the postcard report separately. Otherwise you've built untrackable print), and the destination is mobile-perfect and specific ('scan for an instant quote' → quote form, not homepage).
Best current uses for service businesses: the review-ask card handed at job completion, vehicle and jobsite signage, and EDDM postcards. Free to generate. The value is entirely in where they point and what they're stamped with.