Content re-use is one of the features that you should adopt early in your move from style-based authoring to structured authoring with DITA.
The DITA Maturity Model1 proposed by Michael Priestley and Amber Swope suggests that there are six levels of DITA adoption that an organisation can embrace.
The second level, beyond simply using DITA as a topic format, is characterised by the introduction of re-use. In other words, re-use should be one of the first DITA features that you adopt. A good understanding of re-use, and in particular how the conref (or content reference) idea works, is required for DITA best practice.