Why do you have to practice a skill everyday? You sometimes have the strengthen a skill that you already have mastered. This feature just cost a lot of time and you'll basically have no time left to do a new skill.
You don't have to do anything. If you are sure that you have mastered a skill, you can always choose to move on anyway.
Like elizadeux said, you don't have to do it, but most of us know that feeling of wanting to keep the tree golden. See this neat blog post from a fellow Duolingo user on how to keep your tree golden: http://gregreflects.blogspot.com/2014/12/how-to-make-gold-duolingo-tree.html
That link will change how you do Duolingo forever. It surely changed me forever.
As others have said, you can continue to progress without strengthening your skills.
That said, most of us do not have perfect memories and benefit from repetition of content we have already covered.