having a long-term Interest Vectors, or a set of open problems helps with guiding the "molding" of current projects so they fit into (and help build up) long-term goals
one example is agreeing to give a talk on topic in which you are interested in, but not intimately familiar with yet, so the process of preparing the talk also helps build up understanding (forcing function)
another example would be building small experiments testing small hypotheses orbiting around a bigger theme - of course balance is important here, so when you're using a project to learn new technology, you probably shouldn't also try to do something "wild"