It's simply less expensive in most cases to cache the results of rendering when you render the same glyph over and over. GPUs are fast but not infinitely fast, and they are extremely good at sampling from prerendered textures.

Also it's not just about speed, but power consumption. Once you are fast enough to hit the monitor frame rate then further performance improvements won't improve responsiveness, but you may notice your battery lasting longer. So there's no such thing as "fast enough" when it comes to rendering. You can always benefit from going faster.