Account linking is an awesome feature that lets you play on multiple devices with a single hero.
Say you want to play on your phone on the way to/from work, then play on your tablet at lunch time, and finally play again on your PC during the evening... account linking lets you do this!
Account linking is also a GREAT way to safeguard your progress in the game. Suppose you lost your phone, or it broke? If you have the email address and password from account link, you can restore your progress instantly on a new device.