Available to purchase individually in the in-game Shop's Utility tab, Hero Kit Slots add the convenience of another Hero Kit that allows you to switch between multiple builds quickly. Do you have two builds for a hero that require different stats, items, or Hero Gear? If so, a Hero Kit Slot may be a worthy investment if you find yourself constantly adjusting points and gear to switch between builds or if you prefer to have multiple item and stat builds.
Hero Kit slots can only be purchased via the in-game Shop. A purchased Hero Kit Slot is shared among all your heroes (in other words, you don't need to buy a slot for each hero. Purchasing one Hero Kit Slot allows you to switch between your default and purchased slot on all your heroes.) You can have a maximum of 10 Hero Kit Slots (if you hypothetically had 10 builds for a hero.)
Hero Kit Slots add only convenience and do not give players any other advantage other than just to save time switching between builds quickly. Hero Kit Slots can only be switched before or after a game using the Front End interface, not during (as this would be unfair.) You make the choice before the match starts, and then you are stuck with that choice for that match. Its main draw is it allows you to switch between builds quickly without manually assigning the points each time while waiting for a match to start.
Try Hero mode only chooses the default Hero Kit located in slot 1 if you have multiple Hero Kit Slots. Keep this in mind when testing builds.
For more information on Hero Kits, see Hero Kits.