The idea, is that they are cheap enough, to take a ton.
One on her own? Na, she will die.
3x3 seeker chariot's, accompanied by one herald on an exalted chariot? Some of them are gonna get through, and she can get cover saves behind them.
They are quick enough to almost ensure a turn 2 assault. You will lose some, but those that get through, will tear people up.
That's the theory anyway.
Also, do note that despite being a vehicle, it get's an invulnerable save