Organizing things in themes (or thematizing) is one useful way to give them more value than they have on their own. It is often used in graphic design to produce consistent marketing campaigns based on the various touchpoints between a client and a company. Communication messages are then the same across a variety of different media and their main theme is repeated throughout time and space until it takes a special place in the customer's mind. This is why when we need to think of a solution to our problem, we first determine the category and then ask ourselves which product from it will be the best fit. But if we think in terms of categories, then we might be more sensible to strong themes than to strong products.
Many companies have lots of products, but they rarely fit under a common theme. In this case their portfolio is fragmented, which makes it hard for people to recognize what they stand for. Even a strong product theme can be weakened a lot by diverse product fragments that have few things in common. Clustering the products into various themes often doesn't help too, since having many themes is bound with higher long-term support costs. Even the largest companies will be in trouble if they try to be too many things to too many people. A common theme is such because of its higher priority; developing concurrent ones blurs what is important. It is aligning, because it motivates people to go in the (same) direction of their choice and not of command. It is defining, because choosing the theme will determine the path on the journey. It is distinguishable, because outside people and partners will associate the company with the extent to which it keeps its promises.
Choosing the wrong theme is the quickest way to harm a company and its people. Pursuing something that is hard to do, but in tune with the theme will always be better than doing the easy, quick thing that hurts our initial imagination and our daily need for adherence to it. A stronger theme can build a better defense against external influences over time. It's a good thing, but in order to remain so, the theme shouldn't be suddenly defined by too many concrete products. Even if they (as a group) make it more powerful, they can still take power away, because clients won't easily find out which product best meets their needs. It's a tradeoff that every company has to make.
Common themes can help people find a specific niche and be the best in it. For instance, I have recently seen a site of a logo designer who creates only logos that are round and have crossed arrows in the center. He is so good at it, that every client that needs to have a round logo with arrows is very likely to choose him. The common theme in this case has a selective function. Another example of exceptional work comes from Jessica Hische. She designed the entire alphabet to have these beautiful letters. In their entirety, they have much more weight and are seen as more valuable than if they were only few. For me, it's an example of how a common theme can package a product.