Photos from older phones and WhatsApp
Photos from a 2010-era iPhone or anything that came through WhatsApp got compressed and resized along the way. Upscaling 4× recovers enough sharpness to reprint, post on Instagram, or use in a slideshow without the soft, washed-out look.
Product images for listings
Etsy requires at least 2000 px on the long edge, Amazon prefers 1600 px or above for zoom, and Shopify recommends 2048 × 2048. A 600 px phone shot upscaled 4× lands at 2400 px without a reshoot.
Print at larger sizes
A web-resolution photo (72 DPI) prints clean up to about 4 × 6 inches. For an 8 × 10 you need around 2400 × 3000 pixels at 300 DPI; 4× upscaling gets a typical 1000 px phone photo into that range.
Screenshots and scanned diagrams
A 720p screenshot pasted into a 4K presentation shows every pixel. Upscaling it 2× to 1440p, or 4× to native 4K, keeps text and lines crisp on a projector or shared screen.