It's rude to use things that aren't yours without asking.
Liz has been washing her hands a lot at work to avoid getting sick and now her hands are really dry.
Liz wants to moisturize her dry hands.
Liz sees that her coworker has lotion, so she asks her coworker if she can use some to moisturize her hands.
Liz notices that her coworker has lotion, and she uses some to moisturize her dry hands.
Her coworker tells her that she can and hands her the lotion.
Her coworker sees her use the lotion and tells her that she could have at least asked her.