"Textures have a long way to travel before they are rendered on your screen. FXT1™ reduces their size considerably, thereby reducing bandwidth requirements making the transfer more efficient." - 3dfx FXT1 Presentation

Up to 8 times the texture capacity:

"This table shows the memory requirements for various sizes and color depths of textures without compression. With FXT1™ texture compression, 32-bit images are reduced by a ratio of 8:1 without a perceivable loss in image quality." - 3dfx FXT1 Presentation

More bandwidth available to increase fillrate and framerate:

"This chart shows the relationship between texture bandwidth and fill-rate. Notice the dramatic increase in texture bandwidth needed to achieve a desired fill-rate (4x as much bandwidth is needed when using bilinear filtering!). Without enough available bandwidth, fill rate performance suffers." - 3dfx FXT1 Presentation