Welcome to Android Central.
Depending on your device the means to obtain root varies... Check the forums here for your device and see if there is a guide available.
Reasons... Many users root simply to remove bloatware (carrier/manufacture) apps. Some root to obtain free wireless tethering (carriers charge an extra "hotspot" fee for this feature)
Others root so they can run custom ROMs (customized versions of the OEM OS). This can be a simple few modifications to full blown custom options, themes, and much more.
Last, some root so they can take advantage of root required apps like Titanium Backup or Mybackup Root (to make backups of system files/data/settings and general apps).
Enjoy the forums.