Many brands have parent companies - meaning several different products are owned/sold/managed by one major corporation. In the case of Pepsi Co - they own all the Pepsi, Tropicana, Quaker and Gatorade's sub-brands.
This is important for you to know for two reasons -
1. When looking at an agency roster, when you see Unilever, P&G, Kraft or Campbell's as one of the clients - that may mean the agency works on one or more of the brands that fit under that company. So if you want to - or don't want to - work on something in particular, it's good to know what this all really includes.
2. This helps you understand the importance and the relevance of branding. One company - say Pepsi, can sell something unhealthy like cola, and something healthy like oatmeal, at the same time. But can still keep such separate identities for the two of them that you don't even know they're related. (Kinda how I didn't know Lenny Kravitz and Al Roker are related.)