What does it mean when product lines try to get "the best of both worlds"?

 The Best of Both Worlds When agile development meets Product Line it refers to a group of software development methods in which requirements and solutions evolve through collaboration between self-organizing, cross-functional teams. It promotes adaptive planning, evolutionary development, early delivery, continuous improvement, and encourages rapid and flexible response to change. Systems and software product line engineering, or “product line engineering (PLE)” for short, is a way to engineer a portfolio of related products in an efficient manner, taking full advantage of the products’ similarities while respecting and managing their differences. Considering a portfolio as a single entity to be managed, as opposed to a multitude of separate products to be managed, brings enormous efficiencies in production and maintenance; these efficiencies are delivering order-of-magnitude improvements in engineering cost, time to market, staff productivity, product line scalability, and quality.

It is described by the properties they have in common with each other and the variations that set them apart. The products can comprise any combination of software, systems in which software runs, or non-software systems that have software-representable artifacts (such as requirements, engineering models, or development plans) associated with the engineering process that produces them.

In this context “product” means not only the primary entity being built and delivered, but also all of the artifacts that are produced along with it. Some of these support the engineering process (such as requirements, project plans, design modes, and test cases), while others are delivered alongside the thing being built (such as user manuals, shipping labels, and parts lists).