I often say that a mechanical watch is green technology, no batteries to discard and recycle, and will last a lifetime if properly cared for, spreading it's footprint over many years. However, compared to a quartz watch, there are a few concessions that must be made and these mainly involve timekeeping.
The best COSC chronometer will vary as much as 4.5 seconds per day, so that means a possible variation of 139 seconds in a month, maybe more. Run of the mill quartz movts we tested in watchmaking school also showed about the same drift. However, a mechanical watch is also affected by shocks, sometimes as slight as shaking your watch on it's metal bracelet to adjust the way it lies on your wrist- a common practice.
The oscillator, that is, the balance assembly runs at 4 Hz in a modern wristwatch compared to 32,768 Hz in a quartz. The faster the oscillator, the better the timekeeping. Bulova's Precisionist series is the only significant improvement made to quartz watches, vibrating at 262,144 Hz. The second hands on these watches sweep, reminiscent of the Old Accutron Tuning Fork watches of the 60's and 70's.
Mechanical watches that run at 4 Hz oscillate 28,800 vibrations per hour, and this tops out at 36,000 vph in a so-called Hi-Beat watch. The extra wear and tear of a mechanical mechanism running at this rate negates any benefit gained, so it has largely been abandoned.
Still, this means the quartz watch is more accurate, but we all knew that, unless of course the battery goes dead, or gets too cold or the movt is not thermo-compensated. Batteries leak, too. Generally speaking though, the difference is glaring.
So why would anyone spend hundreds, or maybe thousands of dollars on a mechanical watch?
Longevity is the principle advantage, meaning the possibility of passing the watch down as a working heirloom is fairly certain, if the watch is well cared for.
No batteries to buy, but you do need to service it periodically, and the interval is 4 to 6 years. Back in the early days of the quartz revolution, quartz watches were also serviced, and it is still possible to service some of the more expensive quartz watches, but generally the movement is a throwaway item. A lot of times the cheaper quartz watches are throwaway also, and may be made of very questionable base metals.
Status may be another reason, or perhaps the knowledge that your watch was designed and built with varying amounts of hand work. Patek Philippe famously says you never really own a Patek, you just take care of it for the next generation. Entry level for a new Patek is about $15,000, Rolex is about $8000.
Even if you buy a new throwaway watch every six months, you'd have to buy a lot of them to spend that kind of money, but it needn't be so.
Excellent quality Swiss Mechanical watches like the Hamilton Jazzmaster can be had for under $500, which is about the same as a 50 dollar watch in 1950.
Which brings me back to timekeeping.
Vintage mechanical watches can and do require manual adjustment every few days or once a week. The variation in timekeeping depends on many factors, not the least is how old the watch is and how it was made. 1930's Railroad watches can still run to very exacting standards, for example, but an old watch or clock that needs to get adjusted for a few minutes per week is quite normal, even when they were new. In the case of old clocks especially, the escapement may more often be the culprit here, as certain types keep better time than others, and weight clocks are more accurate than spring clocks.
Adjusting your watch or winding it daily brings you in conscious, mindful contact with your timepiece, a practice that may seem anachronistic to anyone born after the quartz crisis of the 70's.
The main reason people bring an old watch to us for overhaul is invariably because of sentimental reasons, or sometimes because a collector wants to keep his watch in good condition to protect its value.
I still have the watch my Grandfather gave to me when I was a teenager, and I have kept it serviced properly for 40 years. It was already 25 years old when he gave it to me, and drifts a few minutes per week. Every time I adjust the time to the radio signal at 1 pm, I think of him, and the grandchildren who will inherit it from me.