post haste, when used correctly, allows you to push lanes (with heroes that have pushable spells), that either causes the opposing team to try to gank you, which then you tp to another lane to farm, or cause them to just tp back to that lane to stop the push.
I don't give all heroes post haste, such as arachna, maliken, doctor repulsor. But for other heroes that truly benefit from items, like soul stealer, post haste is amazing.
And who doesn't like being able to gank heroes just by tping on one of the wave of creeps?