Adding panels to regions Is there a way where I can use the theme's default regions for displaying panels instead of a layout ?

or Without using page manager, Can I display panes in the sidebar ?
The Mini Panels sub-module of Panels allows you to create panels as blocks, which you could then place into any region on your site. There is an abundance of documentation, tutorials, etc. on using Panels and MiniPanels, but which resources to link you to would depend upon the version of Drupal you are using. There is an existing answer that will probably help you learn more.