Large corporates have a moral and legal responsibility to produce a good a profit as possible for their owners, or shareholders. Therefore, established 'brands' need to constantly consider how they can increase their profits by introducing supplementary product ranges, or sub-brands.
Sub-brands use the strengths of the parent brand as a launchpad to enter a new and often more specific marketplace. A good sub-brand will also have spotted the opportunity from the parent brand. But, it can also be said that a good sub-brand can also create additional market exposure and kudos for the parent brand from its new and exciting innovation. i.e. if one bought snacks from one manufacturer in a supermarket and liked them, you'd be more likely to buy other products from the same manufacturer again. The more products a brand can have in the market place, the better, as its all positive exposure for them.
Source: Wikimedia Commons.
Successful sub-brands include the flavoured Cola ranges from Cola Cola (Cherry, Vanilla etc., as well as their Diet and 'Zero' variations), Renaissance Hotels (the upmarket brand belonging to Marriott) and Channel 4's 'More4', 'E4' and 'Film4'.
Lexus and Toyota are an example of how a sub-brand was created for a specific purpose. Toyota has always had a highly credible and respected reputation for making reliable Japanese Sports and Family cars. But, they didn't really cater for the luxury end of the market. Instead of introducing a range under the Toyota brand, they used the Lexus sub brand instead, which was created during the 1980's to initially create a luxury Sedan model. Lexus now has a range of vehicles under its own brand and is the World's 4th largest luxury automotive brand. Impressive stuff.
With all the Brands mentioned within this article, they all shared the common point, that their sub-brands are recognisable in their own right. Diet Coca Cola is distinguishable worldwide; even though the brand name differs outside much of the UK (it's known elsewhere as Coca Cola Light). But the silver and red branding is consistent.
Introducing a sub-brand does require thought and planning though. The risk is of course, that if the launch bombs, it could actually damage the parent brand. Re-using elements of the original logo is commonplace... but creating a 'need' without admitting the original brand isn't failing or missing something is where the marketing experts earn their money.
There is also a danger of over sub-branding - too much choice from the same brand can actually dilute the exposure, and retailers may be reticent to give away too much of their valuable shelf space to the same overall brand. Sub brands have been known to be unique to some geographic areas only too.