Linux.com - https://www.linux.com/news/what-distributed-system
The simplest application to write and operate is one that runs in one thread on a single processor. If that’s so easy, why on earth do we ever build anything else? Usually because we also want operational and developmental performance. A single server is a physical and organisational limitation on what you can achieve with an application.
Machine Performance (AKA Things)
There are three main operational performance issues introduced by running on a single machine.