There are two stages of fermentation - aerobic (with oxygen) and anaerobic (no oxygen). The Primary is with oxygen, and in the Secondary the goal is without oxygen. If you transfer at 1.05 and DON'T transfer at least some good amount of sediment, you may end up with a stuck fermentation. The sediment contains some of the yeast, some is in suspension yet, but you still want a good yeast population to finish the job!
During the anaerobic fermentation the yeast change their tune, so to speak and produce more of the wine-y stuff you want in the wine, rather than just more alcohol and bubbles. (that's my best technical explanation today, it's been a rough couple weeks at work doing deep dives into finding and then explaining some knotty engineering technical problems...)