You don't need to add oil every 1 to 2 weeks and certainly not as much as a quart. Putting too much oil into the engine is bad and can cause serious damage to the engine.
If you check the oil level, and it is low, then you should top it up. With modern cars such practice is rare. I wouldn't expect a car to need any oil between services unless there was a problem. If you are loosing oil regularly then you need to investigate where the oil is going as you may have a serious problem with the engine. Possibly a gasket needing changed if you see oil drip on the road. Look at the exhaust colour, if it is smoky or especially if it is blue then you may be burning oil which implies worn piston rings which would involve stripping your engine to fix.
As to why you should do an oil change - dirt builds up in the oil over time. Sure after a few days, new oil looks dark, but that is largely cosmetic. Old oil is not simply black coloured but contains large amounts of soot as well as larger items of dirt such a metal shavings. Draining the oil completely washes out all the larger pieces of dirt from the engine - most of which you can't really see but would cause wear on your engine if left in.
Simply topping up is not sufficient as you never wash out the larger dirt. Some people recommend doing a flush after draining where you use a special liquid to clean out the oil sump completely. It is probably a bit OTT for most cars, though. Simply draining and refilling is enough.
A second point is the oil filter. Over time this gets clogged up with gunk and needs replaced. To replace the filter, you have to drain the oil. If you just topped up the oil all the time then you would never replace the filter so all the new oil would just instantly get contaminated by the dirty filter making the whole task pointless.
Thirdly, over time engine oil breaks down. The heat and cold cause the chemicals to change and the oil becomes less effective as a lubricant. Changing the oil at the recommended intervals means you always get the best performance from your engine oil.