I know for myself Layers have been come a large part of what I work with in photoshop since the ability entered photoshop 20+ years ago and the use of being able to Mask objects has only enhanced things further – though using masks can be tricky at times (and I know for myself in the early years I never fully got how to do it, but as the year have gone by its gotten easier to work with). The following site is a good option for those who are looking to learn about Masking it might be semi out of date being published in 2011 but the basics are still there regardless.
Layers are probably the single most important addition to Photoshop since the original version, but layer masks are a close second. I would posit that until you thoroughly understand how and why to use masks, you simply don’t understand the power of Photoshop.
A Complete Beginner’s Guide to Masking in Photoshop – http://designshack.net/articles/graphics/a-complete-beginners-guide-to-masking-in-photoshop/