There are two main reasons to try to pare it down: The processing time, and that <i>if</i> you manage to pare it down to the words that are actually relevant, then you reduce the chance of over-fitting to specific features that are actually irrelevant (e.g. different frequencies of words like &quot;is&quot; is quite likely to be irrelevant; but of course you do this kind of filtering at your peril - what might seem irrelevant could also turn out to be highly significant in context so it&#x27;s hard to get right)