Category 3 wire was originally developed for voice and low speed data applications. Category 5 was originally developed for high speed data, such as 100 Mb/s Ethernet.
However, when it was determined that Category 5 did not reliably support Gigabit Ethernet, Category 5e (enhanced) was developed. Since there was only a small price premium for Category 5e, the market rapidly shifted to the better cable. Although Category 3 can continue to be used for voice, many installers use Category 5e, or even Category 6, for "future-proofing."