When we build Drupal sites we typically have a lot of custom design elements. It is important to be able to display images from the filesystem in a dynamic fashion (non-absolute paths). Should you need to move the site, you wouldn't want to have to reset image paths everywhere. The base_path() and path_to_theme() functions do all of the dirty work for you.

$theme_dir = base_path() . path_to_theme();
print "<img src='{$theme_dir}/_images-base/myimg.png' alt='This is my image'/>";

You may also want to check out the $directory variable that is part of the theme system in Drupal.