Why are the lymph nodes in the armpit removed when a person undergoes surgery for breast cancer?

Metastatsis. The first place that breast cancer will travel is to the lymph nodes in the armpit. A procedure called a sentinel lymph node biopsy removes the first draining lymph node from the breast. If this does not have cancer then we know that most lilkely the cancer has not spread.
Cancer Staging. The lymph nodes are small filters in the body where cancer cells may spread after breaking out of the organ where the cancer started. Removing the lymph nodes gives us a "window into the rest of the body" to help us decide who may benefit from chemotherapy.
Cancer hiding spot. The lymph nodes in the axilla/armpit is usually one of the favorite locations of breast cancer to go to.

Related Questions

What do you think, could having your breasts and lymph nodes removed eliminate your chance of breast cancer?

You would think so. But the answer seems to be only "maybe". Unfortunately there are no big studies. Some women who have genetic predispositon to develop breast cancer elect to have bilateral mastectomy. Read more...
Not entirely. It's fairly common for women with one of the breast cancer family genes to have a prophylactic mastectomy. It reduces but does not eliminate the cancer risk. Leave the lymph nodes alone, you need them. Read more...

Do long lasting swollen lymph nodes in the armpit most likely mean breast cancer?

No. There are many reasons to have swollen lymph nodes. Some are common problems, like infection other are cancerous. Breast cancer is not the only cancer to cause lymph node enlargement. This can only be answered with an examination by a doctor. Read more...
When it comes. To cancer, only looking at cells under the microscope can provide reliable answers. You need a physical exam of your brest and axilla, a mammogram and then a biopsy. Read more...

What are the characteristics of a cancerous armpit lymph node from breast cancer? Hard, immobile, etc?

Not easy to tell. It's common to have a palpable node or two there. Metastatic disease tends to produce large nodes and they may be matted and/or fixed to the chest. If you have no breast mass and the nodes aren't obviously abnormal, I wouldn't worry. Read more...