Nuendo Sample Rates over 300kHz: what are they used for?

Of course. With higher sample rates, more samples pass your DSP plugins in a given time. That means i.e. a brickwall limiter has enough information in shorter time to “know” how to process your audio. The same is true for any plugin that uses upsampling (which is probably any plugin these days?). But don’t mix latency and performance! Higher sample rates usually require higher processing power, so you might need to increase audio buffers which in turn leads to higher latencies.