Like AZ-IPA said there are lots of opinions on this, with many of the more senior members here skipping any time in a secondary. The secondary usually is not properly a "fermentor" unless you are racking onto fruit in which case there will be some fermentation. As the secondary is commonly used in home brewing it is more akin to a bright tank. It basically allows the beer to further clarify and age away from the trub. It is also more frequently used for dry hopping, although there are some who dry hop in the primary, and some who dry hop in the keg.
So it is really a personal preference. I use secondaries, and the primary reason for me is it is designed into my pipeline and lets me do longer aging, while freeing up a primary for another beer.
Also you mentioned a glass carboy, and you will get differing opinions on this as well. Many have swithed to the PET plastic secondaries such as made by better bottles. The PET bottles are a special plastic resistant to oxygen permeatation, and holding flavors. The benefits are they are much lighter, and there is no break hazard to risk cutting yourself. Odd as it may sound the glass carboys can get hairline fractures in them and look fine, then have the bottom just let go one day. There are several stories of serious injury from the glass when this happens. Just a couple weeks ago a member here had the bottom just fall off with a gallon or two of cleaning water in it. He severred a nerve in his wrist seriously enough to require surgery.
