@Riot What is the purpose of Sejuani's Passive?

Frost Armor's application method seems poor for Sejuani and how she's meant to play. As a mainly melee character, Sejuani has a disadvantage against ranged characters. Other than her Ultimate, the only way for her to activate her passive is to get into melee range, but by then you have already succeeded in getting close to a ranged character. Ostensibly, ranged characters are designed so that being close to them means you have them at a disadvantage, so Frost Armor seems like a "win slightly more" passive rather than giving her any tactical advantage. Zac may alter his position to get the best access to his blobs. Rek'Sai may burrow to recover before going back into a fight. Naut's AA's need to be deliberate. Maokai has to have an AA target to get his heal. These passives change how you think abut the space of a fight (while keeping the required action simple). Sejuani's Frost Armor doesn't really enter into the tactics of the player and doesn't seem to do much at any rate. Despite the additional armor you get in the jungle just by hitting things, Sejuani still gets (disproportionately) beat up due to lacking a shield or sustain (or in Shaco & Voli's case, a fear). Shyvanna is the only other jungler that comes to mind without a shield or sustain and she deals with this by burning through the jungle faster (I suspect that's a factor in Sejuani's W change, but anyway).

So what problem was Frost Armor meant to solve? What were the scenarios in which it was supposed to help you most? If you're in range to activate it, you've already accomplished your goal and if someone uses their mobility afterwards, it's unlikely you'll reapply it before you get attacked from range or slowed.