When I joined my current company, there was already a system in place. We're very likely going to rewrite big parts of the system because of changing requirements.
I decided to write up the SAD of the existing system because: it forces me to evaluate whether I understand all the pieces of the existing architecture and why they exist, it will help me find weak- and strong spots in the current architecture, it will serve as a base for the v2 architecture and it will serve as a guide for the new developer coming in soon ( hopefully ).
In order to brush up my skills - I did the 'Developer to Architect' course on PluralSight. It doesn't just go into details about what documents to write and how, but also about which role an architect plays in an enterprise environment.