Bicycle shops brag about their bicycle assembly. Salespeople are notorious for using the argument that bike shops do such a phenomenal job of assembling bicycles to keep you from buying a bike a the local megamart. What makes bike assembly such an art? After all, don't bikes come from the manufacturer largely assembled? Just attach the pedals & the handlebar, and you're good to go!
Well, ehm... no. Bicycles come "pre-assembled" from an assembly line in a country on the other side of the world, whose quality control standards and emphasis may be quite different from our own (We can discuss the politics of this another time.) On our end, bicycle assembly is a very time consuming and skill-intensive activity, and, if performed well it keeps our liability insurance rates down, and, more importantly, ensures your safety.
After removing the bike from it's packaging, we put the bike in a workstand.
First, we work on the wheels. We adjust both hubs, check the dish of the wheel (whether the rim in centered on the hub), correct as needed, and true and tension the wheels. On the rear wheel, we remove the cassette (cogs), grease the freehub, and reinstall the cassette. The tires are then filled to correct pressure.
Next, we attach the handlebars & stem, making sure the brake & shifter cables are being run the right way, and trimming any excess cable & housing as needed.
Then, we remove the cranks & bottom bracket (it's the set of bearings between the crank arms). We grease the threads & reinstall the bottom bracket, the cranks, making sure they are torqued to the correct tightness, and thread in the pedals, also checking for tightness.
We set up the brakes arms & brake pads to the correct angle to achieve "snappy", secure & squeak-free braking, and pre-stress all brake cables. We set up the front and rear derailleur, to ensure that the gears shift correctly, and the chain does not come off the cogs in the event of overshifting. We also pre-stress the derailleur cables.
We adjust the headset (the bearings in the streering column of the bike), and line up & tighten the handlebars and stem. We make sure the brake and shift levers are aligned on the handlebar.
After a final check over, the mechanic test-rides the bike, running through all the gears, testing the brakes, etc. Then he (or she) makes appropriate adjustments. When he thinks the bike is absolutely perfect, he gives it to a senior mechanic for a final check.
It's a long list of steps (we actually sneak in a few more, less important ones), but you should care about this for the following reason: after all, a bicycle is not just a random collection of parts. It is a vehicle, whose performance dependes on a series of complex systems working together in harmony. And your safety depends on that.