Hello Eliano,
Thank you for the question. You probably are not doing anything wrong. :-) Certainly, every photographer has different needs and different styles of working.
As you say, Lightroom can catalog and add IPTC data to photos, but it does so without much speed and efficiency. For photographers who need to browse and cull photos more quickly without waiting for raw rendering and then add metadata, sometimes different metadata profiles, dynamic captioning shortcuts, dynamic file renaming, and then send off photos to a final destination just before deadlines the same day, there exists Photo Mechanic to do all of this and more.
The short answer is that Photo Mechanic is for doing more complex metadata and file management tasks faster than Lightroom alone.
I hope this helps.
-Mick