I think it will vary for person to person - but when I am looking at a new idea(i.e. principle component analysis) that is built on top of old ideas(PCA is based on Singular value decomposition) - familiarity with old ideas makes me more comfortable/confident in thinking that I'll be able to understand the new idea thoroughly and hopefully I'll also see the incoming pitfalls of the new idea. And I really enjoy the process of adding new concept/idea in an existing larger picture.