function _e and __

Defined in wp-l10n.php

// Return a translated string.
function __($text, $domain = ‘default’) {
global $l10n;

if (isset($l10n[$domain])) {
return $l10n[$domain]->translate($text);
} else {
return $text;
}
}

// Echo a translated string.
function _e($text, $domain = ‘default’) {
global $l10n;

if (isset($l10n[$domain])) {
echo $l10n[$domain]->translate($text);
} else {
echo $text;
}
}

Used all over the place in the WordPress code…. but what purpose do they serve exactly?

Thanks for any help! ??