Building a computer 
Pros:
* Cheaper in the long run
* Easier to fix
* Better overall quality

Cons:
* Can be more expensive upfront
* Higher chance of user error
* You need to figure out part compatibility

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Off-the-Shelf computer
Pros:
* Faster to buy than to build
* Plug and Play
* Normally Cheaper to buy rather than build

Cons:
* Repairs are harder to do
* Build Quality can be lower
* Limited configuration available

There are plenty of other reasons that can influence your decisions but it comes down to how soon you need a computer, and how confident you are working on a computer.
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