Perfectly crafted ice balls are what separates a simple, cheap drink from a professional pour. Sure, the bartender could have placed a few chunks of ice into a glass, then poured the whisky on top but that would’ve been a different experience. Think about how quickly the ice would have melted with individual ‘chunks’ of ice.
By contrast, the handcrafted ice ball melts slowly, allowing one to savor the slowly changing flavors. Indeed, even one drop of water has a big impact on a whisky straight so it makes sense to slow the evaporation process as much as possible.
Usquaebach Reserve is recommended on the rocks but to each his own.