This ppi issue confuses the most professional amongst graphic designers.
ppi means "pixels per inch" and is needed to define the pixel resolution in reference to an images physical size on an output device. e.g. A 3000px wide image at 300ppi is 10 inches wide when printed. Change that to 150ppi and the image will be printed at 20 inches even though it still has 3000px.
When resizing for screen the ppi value becomes irrelevant and can be neglected, because screens come in different sizes and resolutions. Here the pixel value is key to define a size. 72ppi is just a default value because it has to say something, and older screens average around that value.