There really is no secondary fermentation (short answer for what most of us brew). So - you can leave it in the primary for a month and it won't hurt anything. (sitting on the yeast won't hurt, although many of us were originally taught that - turns out that's fiction).
Why use a secondary fermentor?
First, because the beer will be clearer when you bottle it, so there will be less sediment in the bottles. This is because the beer has a chance to settle again in secondary. There will always be a little bit of sediment in the bottles when using the natural carbonation methods popular with home brewers.
Second, because fermentation will be faster and more complete when using two fermentors.
Fermentation starts out quite vigorously, and then gradually tapers to a stop for a few days. After the peak of fermentation, inactive “spent” yeast settles to the bottom of the fermentor. This inactive yeast actually inhibits the yeast that is still active and trying to finish the fermentation.
When you “rack” your beer (transfer to a secondary fermentor) you leave the inactive yeast behind, and the physical motion of the beer drives some of the dissolved CO2 out of solution. This re-invigorates the fermentation, so that it finishes faster, and the beer will have fewer off-flavors. This is more important with very strong beers, and with many lagers.
Third, because you can leave your beer in secondary much longer than you can in primary.
If you leave your beer in primary for more than 2 weeks, all of the settled yeast starts to “autolyse”, which means the stronger yeast cells start cannibalizing the weaker ones. This gives you harsh yeast “bite” in the finished beer. But when you rack to secondary, most of the yeast gets left behind, so it is safe to let the beer settle and age after fermentation is complete.
This is useful for stronger beers that need aging to mellow their flavor, and for making lagers, in fact, the word “lager” means “to lay down” (i.e.: to age). http://www.annapolishomebrew.com/REsecondary.asp
Yeast bite acts differently depending on the type of yeast. Most brewers yeast will not autolyse fast or even seem to at all. But if you don't know why take the chance?