Many people take the manufactured lids off their tanks for two main reasons: first, they want to replace the stock lighting in these all-in-one tanks with a custom setup, and two, because of the current fashion for open or rimless-like tanks. There are multiple other reasons for removing a canopy, and I think many reefers enjoy DIYing and modding their setups, and so bulky canopies are often the first things to go. Unless you have a tank in an environment with very low ambient oxygen, or are keeping fiddly and difficult SPS, though, it's just a matter of choice. I keep my lids on because it really does reduce evaporation, which is a major issue in my dry climate.