By default Google Analytics loads with a daily graph view for the last 30 days, and you can see a day by day evolution of your traffic. Starting today there are two more graph views added, that show the traffic evolution by week and by month. These two new options are useful when reporting data over a longer period of time (say one year), because in the daily view the graph would look very crowded and you wouldn't be able to see the trend.
If you're asking me, the best reporting feature Google Analytics has is the fact that you can do a comparison between a current period and a previous period of time, this way you can see how things changed from the previous period - this doesn't limit to the visitors graph, you can also see: how visitors sent by referrers changed, content visitors trend, search engine referrers trend, ...
One problem is with the Google Analytics Comparison view, the fact that you must be sure you select for the two periods compared the same starting day. Why? Well, during week-ends the traffic goes down compared to the other week-days, so if you're comparing how traffic trend is between a period comprised of Monday, Tuesday, Wednesday, with a period comprised of Friday, Saturday, Sunday is clear that the trend will be ascending, which is not correct because you should compare the same days. You can see in the graph below how the trend seems ascending but this is because one period contains the week-ends: