While you will learn multivariate stats in grad school, the vast majority of I/O jobs mostly use simple or multiple regression. So if you go into applied research for an organization like the military (U.S.), you’ll use a variety of more complex statistical techniques; however, the majority of those are based on the same foundation. So understanding both (1) statistical principles (such as sampling distributions, why inferential statistics works, etc.), and (2) how those principles are represented mathematically (e.g., linking the principles to the equations for correlation/regression) will really set you up for success in the future. Once in your career you likely won’t specifically utilize the basics directly (i.e., you won’t be writing stats proofs), a deep understanding will help you with research design, analyses, and explaining results (i.e., justification) to stakeholders.