That it is pegged to the USD is mostly irrelevant. The main purpose, as @blocktrades noted in the other reply, is to maintain relatively stable purchasing power. For example, it is used by the DHF to set hourly payments for funded proposals which go on for an extended period of time. We don't want someone being paid to work for Hive to see their pay increase or decrease by 10x or even 100x just because the price changes. That's highly disruptive and damaging.
If, for some reason, USD itself were to go haywire and stop maintaining relatively stable purchasing power, we could decide to start pegging to something else like a different currency or a basket of commodities. Either way, the mechanism for having a stablecoin that maintains a peg still needs to work.