Why does WordPress have more than one salt?

Question

I guess I am exposing some lack of security knowledge here, but shouldn't it be enough to have one salt? Why are four different salts necessary?

define('AUTH_SALT',        'z(ly|p-aeKf^I~OfOUUIL&Y?C5Z.iu|L}kY%dvclq.h9n`)MlZe6');
define('SECURE_AUTH_SALT', 'NIY8g>=l9y~eV~WLu 3n>UG#3wSl4YfT%;z9`7m9Gk/k_Vn4`ej8');
define('LOGGED_IN_SALT',   '<-[R@, I;m%n*9G?CU1a:))pEAa/r5X5@pT`cO2H|c2&x~G<p*3T:-5v<N');
define('NONCE_SALT',       'm(-0:+r0a%z~a:2F;]-geM$9~!4j(q3QdpkmB7;P+ZYYw7Rdy{97fS');

(Don't worry, values mangled and it is a local site.)

Answer 1

I posted the question on the wordpress slack and the answer I got https://wordpress.slack.com/archives/core/p1447905339001506 is that the difference is semantic and not practical and there is no reason to prefer the usage of one over the others (except for semantics of course)