Why?

Because you should be using your drones! Short Reach balances the gameplay away from manual labor and helps emphasize automation.

How?

It sets several properties on the Character as documented https://wiki.factorio.com/Prototype/Character. This is distinct from the approach used by Far Reach, which allows you to increase the "bonus" distance. This means that Short Reach and Far Reach can be used in conjunction! You can use Short Reach to reduce your base length to 0, and then use Far Reach to increase the "bonus" distance by as much as you like. Added together, you can get any distance you like.

MIT Licesnse blah blah blah