Why is manga typically created with screentones rather than grayscale fills? I was recently introduced to the screen-tone process. Here's a short video for reference. Why was this, and why is this still the prominent technique for coloring manga? What advantages do screen-tones have over grayscale fills?
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To clarify, a fill is when an artist uses a solid color to completely color one area (with marker, paint, pencil, etc...).
These are called Ben-Day dots. Depending on the size and spacing of the dots, you can create different shades using a single colour - or, with a limited colour palette you can produce different shades not available to you.



This is often used to save on printing costs, as you can get a wide range of shades/colours with a smaller usage of ink and no need for different levels of pressure.

Because manga is often a disposable medium, keeping costs low is often a priority for magazines and such to pull a profit - without increasing the prices for consumers.

You may see this in other non-manga print mediums as well - like free/low-cost newspapers:


It has become associated with comics both East and West - so often shows will use it for artistic purposes as well.



Other screentones exists, like speed markings, shoujo sparkles and such - and simply, they are used because it is both easier and faster to get your desired effect.

As well as providing particular aesthetics, it ensures a colour is solidly evenly shaded, whereas when colouring manually it is easy to have uneven shades by angling or pressuring differently across an image.