Heirloom gear has two uses 1) provide the character with solid statted gear while they level and 2) the ability to pass the gear on to alts once the character hits level 40 (usually via Hideouts using the Shared Stash).
There are other types of gear that has the same "grows as the character levels up" effect, but bind to the character instead of account, but once the character hits 40 the secondaries are inferior to any level 40 purple that benefit your Super Stats and the primaries are inferior to level 40 blues.
Comments
You can use it at level 40, but at that point you should be working on gear that allows you to use MODs.
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"Okay, you're DEAD, what do you do NEXT?"
There are other types of gear that has the same "grows as the character levels up" effect, but bind to the character instead of account, but once the character hits 40 the secondaries are inferior to any level 40 purple that benefit your Super Stats and the primaries are inferior to level 40 blues.