To quote..
Estrogen : The first hormone we need to keep an eye on. Many AAS convert to  estrogen via the aromatization process. Some AAS are worse than others. Also, estrogen spikes after a  cycle. High levels of  estrogen leads to  gyno, water retention, fat storage etc. Estrogen plays a key role in progesterone related  gyno. We either block its receptors with SERMS or reduce its production with AIs. We watch  estrogen levels during a  cycle and in  PCT. Lowering  estrogen too much will mess up your blood lipids. Letting it get out of control will cause sides like  gyno, water retention etc. Estrogen plays a role in  IGF-1 levels, may lower  IGF-1 when blocked with a SERM. Estrogen is also beneficial hormone when bulking, promoting higher androgen receptor concentrations (!). 
It also is beneficial in another way - its supposed to act as an anti-inflammatory - this means blocking or reducing it too much during a heavy bulking  cycle can result in injury to joints. Obviously different  estrogen levels are desired for different goals, and it is not always good to block its action or its production. Usually, while bulking, estrogen is allowed to rise unless  gyno or water retention (leading to high blood pressure) becomes a problem. When cutting and shedding water and lifting a little lighter (contest prep for example) estrogen is usually dropped with an AI. Proper  diet and training can help the bad side effects high  estrogen can have.
From
http://forums.steroid.com/showthread.php?t=198848