Pixels are normally presented on a computer screen at the rate of 72 every inch in both directions (the "resolution"), while a document intended for printing on paper might have 150, 300 or many more dots per inch.
You could of course keep all your files at a high resolution so they work both on screen and for print: but for any documents which will only be seen on screen, it's an enormous waste of resources, both storage space, and computer power to display it.
E.g., an uncompressed image 1" square for screen viewing is 72x72 = 5184 pixels; for economy print at 150x150 dots/inch (dpi, roughly the same as pixels/inch, ppi) = 22,
500, for normal print at 300x300 = 90,000, and so on: so a file for normal print can be nearly 20 times bigger than one for viewing on screen. Of course in practice various compressions are used, but the difference is still big enough to matter.So it's normal to think in terms of two parallel versions of things, one for screen, one for print. That's an oversimplification too, of course, since there are many different qualities in printing, not to mention methods.
Another reason for keeping the distinction clear between screen and print, though it affects the using of fonts rather than the making of them, is that the two processes use colour in different ways: colour on screen is made up of various combinations of red, green and blue - RGB - while coloured printing inks are made from various combinations of cyan (turquoise), magenta, yellow and black - CMYK.
