Predictive completion is only as good as the dictionary it uses. The dictionary doesn't only list the words themselves, it also ranks them according to how likely they are, so that predictive mode can offer the most likely completions first.
As you type, predictive mode learns which words you use more frequently, so that the predictions improve. It can automatically ensure certain words are always ranked higher than others (useful e.g. when one word is a prefix for another).
Predictive mode is not restricted to using one dictionary at a time; it can use many dictionaries in parallel, and can automatically switch dictionaries in different regions of text, the regions being defined by regular expressions.
And predictive mode attempts to do all of that faster than you type, so that your typing is not slowed down even when using very large dictionaries. (As soon as Emacs becomes sentient – surely not far off – it will probably go on strike through being forced to work too fast!)