The goal with letsencrypt is to get everyone using SSL, even the layman. With that in mind they aim to automate the process as much as possible so you don't need to remember to update your cert or worry about it expiring. The client will handle all this for you in the background.

They are even considering shortening them to less than 3 months. I think the goal there is to ensure people automate it who would otherwise try and do it manually.