The better the weights in a dictionary match the frequency with which you use words, the more useful predictive mode will be. Some of the standard dictionaries already include word weights, which match average word frequencies taken from a large sample of texts. Some don't include any word weights. In any case, your personal word usage might be very different from the average.
Ideally, the weights in a dictionary should match your personal style of writing. In fact, since your writing style might change significantly depending on whether you're writing, say, a scientific article or an email, it may even be worth having different dictionaries for different circumstances.
The easiest way to teach a dictionary about your writing style is to supply it with samples of your writing, and have it learn the word weights from them. Predictive mode provides two ways to do this: learning from existing files, and automatic learning as you type.