I find the Week and Month tabs on the home page of stackoverflow.com to be indispensable for how I use the site. In particular, most of the time that I browse the site I want to find interesting questions and answers so that I can learn from them. However, I am not able to spend much time on the site so I browse it once or twice a week for a relatively brief time. The Week and Month tabs allow me to quickly find interesting content that I would not have time to find by browsing through thousands of questions.
Now, I understand that these tabs do not necessarily show the most interesting questions but instead show the most popular questions, which might be the questions that have been bumped several times, received an answer from a high profile member of the community, or are about a common as opposed to novel issue. These tabs are not perfect; nonetheless, they go a long way toward helping me quickly pick interesting questions out of the vast sea of noise.
Unfortunately, many Stack Exchange sites, such as mathoverflow.net, programmers.stackexchange.com and politics.stackexchange.com, don't have these tabs. Fortunately, I did notice that math.stackexchange.com recently acquired them so perhaps there is already a push to add them throughout Stack Exchange. If not, I kindly request addition of these tabs to the sites that don't have them.