It does need to be booted up and awake, which hasn't been an issue for me as I have that computer running continuously for other reasons too (like running an Airsonic media server). But there's some things you can do to have it wakeable when you try to connect: https://www.reddit.com/r/Steam_Link/comments/3r9sv7/using_the_link_with_computer_set_to_sleep/