Uncompressed TGA is used for two reasons: Quality (this particular image was going on a screen three stories tall) and, as the other commenter mentioned, the time required to decompress images.
I'm not an expert on the exact architecture, but I'm guessing that with uncompressed TGA you just throw the bits at the GPU and they get displayed, while if you have to uncompress images that first gets handled by the CPU (?).
I'm not an expert on the exact architecture, but I'm guessing that with uncompressed TGA you just throw the bits at the GPU and they get displayed, while if you have to uncompress images that first gets handled by the CPU (?).