The Design Update: Borders around Text and images

I'm designing a slide for a presentation and was given the feedback on a series of text boxes and images that the black border around them was heavy/unnecessary. So I was recommended to either use a light grey or (preferably) no border at all.

I want to know why this is preferred? I don't have a very good eye for design, so I'm just curious what the thinking is behind these borders, or lack of them.

Thank you!