Simply the closer the network route, the lower the latency of the connection. Which means your miner receives work and submits competed work (shares) faster, reducing the number of outdated (stale) work you submit. Typically what you want is less than 100 ms.
https://www.youtube.com/watch?v=QnHY5owtkHAI'm sure if they thought about it they would realize its this way with a lot of thing that are dependent on a good low latency, gaming for instance is the same way, if you want to dominate you need to have a low latency/ping which does affect gameplay much like it affects our mining, further away we are from the pool the worse the connection to the pool is, the closer we are, the better it is