The main reason you'll want to download this update sooner rather than later is that it brings the new Photos app that is replacing iPhoto. iCloud Photo Library is also now out of beta, and these updates will work with the new iOS 8.3 update for your iPhone and iPad.
Federico Viticci, writing on his site MacStories:
There have been many cautionary tales about Apple's cloud services and photo management apps, but I think they nailed it this time. I'm happy with iCloud Photo Library because it's seamlessly integrated with my iPhone's camera and photos – I don't have to manually upload anything, and I don't have to think about managing photos.
I just finished upgrading and I'm waiting for all my photos to be uploaded from the new Photos app to iCloud Photo Library.