Let's say you have natural gas installed at your house. You call me to run a gas line to that brand-new fancy Vulcan gas range you and your wife just bought, that you saw on "Extreme Makeover". When I come there to quote the job, I can just pipe up the range, do it cheap and quick, and forget to mention...that for a few more $$$, I can rough in large enough pipe/tees/valves to take care of any future need you may have...want a gas grill, dryer, instantaneous hot water heater, gas logs/fireplace for those romantic evenings? Or, a gas light in the front yard. Just hook it up and you're set.
Instead, if you choose only to run the line to the range, then call me back 6 mo. later to put those gas logs in,(remember the romance part of that?) I'll have to upsize the entire line, and rip out what I had already installed and charged you for. You would be a little preturbed, wouldn't you?...... "Why didn't you put in the bigger line in the beginning?"
Is that giving away work? Different scenarios, but the same principle.