In what situation command prompt appears instead of a dialog boxes?

When ever you will write the command with Hyphen (-) means before writing the command put (-) hyphen then command prompt will appear all the time. for example if you write the "LA" or Layer in command area the dialog box will appear but if you write the -la or -layer than dialog box will not appear only option will appear in command prompt area.

If a file command is imported from a script or AutoLISP/ ObjectARX/ ADSRX a command

prompt appears instead of a dialog box.

If you use scripts, you can run a command by displaying prompts on the command line instead of using a dialog box.