Why do some icon themes have so many different sizes?

[12:24 PM] /usr/share/icons/hicolor $ ls
128x128  192x192  24x24    32x32  48x48    64x64  96x96             index.theme
16x16    22x22    256x256  36x36  512x512  72x72  icon-theme.cache  scalable
[12:24 PM] /usr/share/icons/hicolor $ 

By trial and error, I've figured out that 16x16, 22x22, 24x24, and 48x48 icons are used in Lubuntu 13.04. I may have seen icons corresponding to 64x64 and 128x128 being used but I'm not sure. But what are all the other sizes for? Are they needed for very large screens?

Why do some icon themes have so many different sizes?

Software often makes use of different icons thus it needs icons to be in different sizes. The difference in sizes can be attributed to the context within the software they're used, for example: Icons for the toolbars don't need to be at 512x512 whereas icons in a dock or menu need to be at a higher size.

Also, icons need to be scaled properly so they don't lose their meaning.

So artists make use of the different sizes for this. An icon created in 512x512 will look good at 100% scaling and probably at 50% but at 12.5% (64x64) the same icon will look bad, details will be lost, it may look like something completely different, etc; so another icon specifically made for that size needs to be created.

Looking around I found that Gnome has a specification for the sizes:

I'd add to this that this is specifically for Gnome icons (mostly in SVG) and maybe the Tango and Tango-based icons though the table looks quite dated as Gnome includes icons larger than that nowadays and doesn't mention symbolic icons.

So depending on the artist these may be very well vary.

I generally group icons in the following sizes (size of the canvas) according to the context in which they will be used:

• Contextual Menus, Toolbars, Mimetypes (very small), Mounted Devices (Nautilus sidebar).

  • 16x16
  • 22x22
  • 24x24

• Mimetypes (small-medium), Docks, Categorized Menus (Unity's Dash, Gnome Shell's application overview, System preferences), Desktop.

  • 32x32
  • 48x48
  • 64x64
  • 96x96

• Mimetypes (large), Docks, Categorized Menus (Unity's Dash, Gnome Shell's application overview)

  • 128x128
  • 256x256
  • 512x512

The icons are used by multiple features in the unity system, and in all desktop environments, each choosing the size of the icon best suitable.

The sidebar, for example, uses a smaller icon than used by the application switcher (pressing alt+tab shows very large icons, it uses 256x256).