For Fun: Sometimes modifications can simply be for fun.This can include texture packs to change the colors for the color blind or special packs for those with difficulty seeing properly in the dark.
That being said, there are various reasons to modify your experience using Minecraft texture packs, including: Sometimes, when playing a game that is designed to give you unlimited tasks and world-building abilities, spicing it up with mods and changes can keep things fun and interesting for more extended periods of time between expansion packs and DLC.
