Check mark in upper left hand corner of an icon

Why do I get a check mark in the upper left hand corner whenever I click an icon?  Why do I get a check mark in a box to the right whenever I click on a folder?  Why do I get a check mark in a box to the right whenever I click on a file?  What is the purpose of these check marks?

James

In Windows Explorer you can select files and folders by ticking the boxes alongside the names. This is an improvement on using keyboard shortcuts like Ctrl+A or holding down the Ctrl or Shift keys and clicking on another file/folder to select them. Also, the files/folders don't have to be displayed consecutively, i.e. the first, third and tenth can be selected.

It is enabled/disabled via Folder Options > View Tab and putting/removing a tick in Use check boxes to select items.