Different jobs
A logo identifies a brand across many contexts — websites, business cards, ads, merch — often horizontally, with a wordmark, and in flexible sizes. An app icon does one job: earn a tap in a tiny, fixed, rounded square on a crowded Home Screen. Because the jobs differ, the best solution for each usually differs too.
Shape, detail and text
Logos can be wide, detailed and include the company name. App icons must be square, simple enough to read at 40px, and free of text — the app name already appears underneath. A detailed horizontal logo crammed into a square almost always looks cluttered and illegible at icon size.
Do you need both?
Usually, yes. Many brands derive their icon from their logo — extracting the symbol or monogram, dropping the wordmark, and adding depth or a background so it holds up small. The icon and logo should feel related, but each is optimized for where it lives. If you only have a logo, treat the icon as a focused adaptation, not a straight copy.