I think one of the main reasons to have any sort of daily/weekly, along with the same reason as having bounties, is to have players play content that may not normally be played. By changing it up every day/week, it rotates the content and makes it feel less like grinding, albeit by a small amount. As EagleEye 989 and BlackjackHD have said, it's also good to farm Ascendant materials with, too, with the higher difficulty. Always check out what higher tier missions/strike reward you with to compare the reward with the challenge. :)