Let me add to Isabelle's comment. At a more basic level, much of the work inside the Investment Banking Department involves understanding a client company's operations and then making suggestions regarding the financial elements of its business. For example, if you were advising McDonald's, you would be expected to understand and develop a spreadsheet that demonstrates how the company generates sales and what types of costs are involved in operating the business. You would also need to understand who are McDonald's customers and who supplies the company with food and supplies. In essence, you would study the entire business.
From there, you would help McDonald's address its concerns. Perhaps they want to understand how to raise money to pay for hundreds of new stores in Asia. For that, you might suggest issuing new stock or bonds. If they agree, your team would help find investors who want to buy those stocks or bonds. Or maybe they want to get into the growing sandwich market. You might discuss what would happen if McDonald's bought the entire Panera Bread Company or Jimmy John's.
All of these elements involve finance, but the great part of investment banking is you start by understanding what a company does in the first place.