(conversation in #cameratalk on 2015-11-18 & 2015-11-19)
DPI/PPI is important any time you’re crossing the physical/digital boundary – in either direction.
When scanning, you’re going from inches to pixels, and when you’re printing, you’re going from pixels to inches. DPI matters in both circumstances. Or, more specifically PPI, but DPI is commonly used to mean pixels per inch.