This helps when you're facing stealthed units such as Rogues.
The stealth detection increase is +30, which is the same as 6 levels. This means a level 64 hunter can detect a level 70 rogue at the same distance as a level 70 that doesn't have stealth detection increasing effects.
This is immensely useful in PvP. In a battleground, if you hear that a Rogue is nearby turn on Track Hidden to expose it as quickly as possible.
Can be activated along with other tracking abilities, like Track Humanoids.
This only increases your stealth detection and places the mobs YOU DETECT on your minimap. If a mob is much higher level than you and stealthed, it is likely you will fail to detect the mob before the mob has a chance to attack you.