Well, when you take a 1024x1024 TGA texture, its quite BIG (~3 MB only RGB color, 4MB with alpha channel). If you save it with RLE compression, you can save HD space, though, inside your graphics card there is no real difference. You can actually force A6 to compress ALL textures, but this would be worse: it has to load those big texture to compress it again. It is actually faster to load an already compressed texture with precalculated mipmaps. In addition, DDS textures are in most cases (even with precalculated mipmaps) smaller than its TGA counterpart. There are a lot of tools around to create DDS texture. I recommend the ATI compressonator for solid textures and the NVidia toolchain for RGBA textures (this has a reason: while accepting the solid RGB DDS files composed with the ATI compressonator, WED/MED and A6 raise directX errors when displaying/rendering them. I had no problems when I compiled RGBA DDS files with the NVidia tools.

Decreasing memory consumption and loading times are suitable especially for games or levels with a great amount of massive graphics (big textures, fine grained skins, shaders, terrains, panels (as well!), etc.).