It was only when Siddhartha stopped being afraid of pleasure dissociated from sensuality that he entered upon the path to enlightenment. For it is not by austerity but by happiness that one finds nirvana: "There is no way to happiness; happiness ["pleasure" (sukha) as a factor of absorption] is the way!"
(More happy panda pictures at Kjdrill/Flickr)
It purifies the mind-heart and stabilizes mindfulness. It intensifies the mind making it manifest what it is capable of, things we ourselves would not believe of it.
For example, when coming out of a meditative absorption it is very useful to review the factors of absorption associated with it. In this way one ensures where in the practice one is. This is done by turning attention toward the "mind door" which is located in the area of the physical heart. Without absorption (or at least access concentration), one sees nothing.
But with it one notices something that has always been there: a greenish mirror reflecting whatever is in the mind. One can then check the factors. (Only do this under a teacher skilled in the absorptions for, as the Gavi Sutra or "Cow Discourse" points out, one foolishly loses even the absorption one has already gained to say nothing of going no higher).