My mom uses my Prime account for shipping. I set her up through the households thing so it is linked to her account. And sometimes when I am at her house and she wants help ordering something it is easier to just do it on my phone or tablet and have her transfer me the money.
A couple of years ago my sister borrowed my login for Prime Day but didn’t end up ordering anything. She got her own account since then but still used mine a couple of times to order things without her husband knowing before they separated.