Ideally you want to max it and use it to secure ranged creeps on lane, as well as bounty runes.
Your bread and butter as a support/utility Bloodseeker. You don't need it early on, but when you get it, it gives your core an equivalent of a Hyperstone
Your early game surviveability. You don't even need to be good at last hitting, all you need to survive - to be around the creeps, and optionally you can be as annoying as possible
Core
A good alternative to Arcane Boots if you just want to build into early aggression
Situational Early
Lategame
Allows you to practically immobilize two targets in fights
~15-20 min
ORDER SWAP: Maelstrom
Works great in combination with Bloodrage
Situational
Additional movement speed and mana regen are already nice for Bloodseeker, but what makes it even better is the ability to cancel teleports and setup Bloodrite damage with it
Other Items
~16 min
ORDER SWAP: Black King Bar