The short answer
Nearby developments can affect views, light, noise, traffic, privacy, and long-term value, even if they're not next door.
A planning application a few streets away can still change what it's like to live somewhere, or what the property is worth when you come to sell. It's easy to miss because it doesn't show up on a viewing.
Why this matters for your purchase
Every PropM8 report checks the planning constraints section for exactly this: nearby applications and decisions that could affect the property, not just what's on the plot itself.