Basically, Google shows the autocomplete suggestions whenever you start typing anything into Google search box. It is in Google's best interest to show the most relevant keywords in the autocomplete suggestions. Keywords that would help Google to retrieve the most relevant websites and help users find the most relevant content for their search query.
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).