There are a variety of reasons you would need professional bike assembly:
~ You found a bargain online that will be shipped to you but don't have the time to assemble it your self or would prefer to have someone else do it to insure the job is completed properly.
~ You have been travelling with your bike for vacation or an event and need it reassembled after shipping it home.
~You found a great deal on a collectible frame set on craigslist and have a period-correct build kit to put on it and want it done right.
The proper assembly of your bicycle - according to manufacturer specifications - is the most important factor in assuring it consistently performs and lasts a long time. Correct torque values, bearing adjustment, spoke tension, and simple pride in workmanship all work together to deliver an enjoyable riding experience for many years.