Preparing the Mishkan (Tabernacle) for transport required covering all the utensils. While most of the items were covered by the tachash (4:8,10,12,14), a beautiful and durable covering, the Aron was covered by both the tachash and then techeilet (a blue woven tapestry) on top of it (4:6). Why was the Aron covered differently, and why did the techeilet specifically cover it?
Rav Moshe Feinstein explains that the two types of covers had different but complementary purposes. While the tachash cover was beautiful, the techeilet was meant to remind its viewers of heaven and, ultimately, the ultimate purpose of its contents. While adorning the utensils with beauty may seem appropriate, the cover for the Aron adds a qualification to all the coverings: Be mindful of the purpose of your actions. While beauty for its own sake is surely aesthetically pleasing, beauty with purpose uplifts and protects.