Why do computers have transformers on the power cords?

Most desktop computers don't have the Transformers located in the power cord themselves. The location of a transformer in a regular desktop computer is inside a component called the 'Power Supply Unit' (or PSU). The PSU is the part of the computer that the cord plugs into, and it typically has a fan to provide cooling to itself. It is due to this nature that you can typically swap out the 3-pronged computer or monitor cords without much concern.

Notebook computers, on the other hand, have their transformers located outside of the computer body, and embedded into the power cord. There are a multitude of practical reasons for this.

1) Notebooks can get very hot. Heat can degrade the performance of the transformer, causing it to output less wattage than it normally would and degrade its life span. And the heat that the transformer can generate can also degrade the performance and lifespan of the notebook.

2) The transformer is typically very big and would not fit inside of a notebook without making it very large.

3) Power transformers have a fairly high rate of failure (compared to some other parts), so their easily replacable nature makes it a good idea to keep it on the outside.