Estimating a trend line, basic question

Basic question here, suppose you are looking at annual trends for sales and you have complete records for 2005 - 2008 and for the 2009 record the year is obviously not over yet, only 8 months in.

Lets say we were interested in total sales for the year. I was told you could "estimate" the general trend line,or estimate of the sum of sales for the year, by taking the total number of sales thus far for 2009/8 (January through last present record of august) * 12 (total months in year)= estimate of 2009's trend line.

I was told doing so would give a decent educated estimate as to what to expect the trend line to look like for 2009. Could some one please explain why this works, if it even does?

Many thanks