The texture asset window shows how much video memory your texture is going to use at the current compression/mipmap settings in real time. While mip maps do increase memory usage by around 30%, they drastically improve performance if your texture is ever displayed at any angle/distance where its shown at under half its size. That’s because when you go under that size, the GPU needs to start skipping texels while displaying the texture (the more zoomed out, the more texels it needs to skip), which means there will be more cache misses, forcing the GPU to access memory more often.
The UserInterface2D preset means uncompressed, BTW. Since UI elements are often displayed without perspective and at near 1:1 texel/pixel ratio, they can do without mipmaps. UI textures are also often not power-of-two sized, which is not compatible with mip maps and most block-based GPU texture formats.