Spatial Memory

From Wikipedia:

In cognitive psychology and neuroscience, spatial memory is the part of memory responsible for recording information about one's environment and its spatial orientation. For example, a person's spatial memory is required in order to navigate around a familiar city, just as a rat's spatial memory is needed to learn the location of food at the end of a maze. It is often argued that a person's, or an animal's, spatial memories are summarised in a cognitive map.

This is very important stuff to consider when you design User Interface interaction.

Think to your bench when you drop a pen or a note. When you need it, you'll find it without thinking (and if you're right-hand usually you organize stuff on your right). That's spatial memory!

When you design user interactions, all state-driven command (navigation tree, state information, ..-) should be kept on the right side of your screen (well that's not true for left-handed people and you should have a configuration option).

So why in every application the commands are on the left?

Because when the developers begin the implementation, first they think to the commands menu and menu-bar plumbing them on the left (...you read from left 2 right) and after that he will think to the "application meat", that will fit the right area.

The application will leverage "the dev way of thinking" not the spatial memory.