Have you ever tried to select an object, then start a command to modify it, only to find that your object had de-selected itself and you had to pick it again?
The culprit was likely the "noun/verb" selection mode, located on the Selection tab of the Options dialog. (It's also known as PICKFIRST, if you prefer the command line.) When that box is unchecked, or when the variable is set to 0, you have to start a command before you can select objects.
These days, most people leave it turned on, but it's mainly a matter of personal preference.