Possessing general knowledge is indispensable, especially for market sizing cases
In a case interview, you must be familiar with basic data about geographies, population, life expectancy, and economies especially for the country you are interviewing for and popular countries such as the US where most big consulting firms are based. Having this knowledge demonstrates that you are well-read and have the potential to be business savvy. Thus, besides being able to solve highly complicated algebraic problems, you should know basic information such as the population of your home country and some key countries of the world. Obviously, it would be embarrassing to not know the population of the country you live in, especially if your client notices this!
Most importantly, you will likely need this information for most market sizing cases, and backing assumptions with known facts such as population data makes the entire exercise of estimation much more plausible and reliable. Knowing such figures can also be beneficial for sanity checks at the end.
Here are some basic facts we pulled together for you (ideally you should memorize these or at least have an idea of the "order" if not exact numbers):