Perhaps most importantly, the Rolex watch's impeccable reputation depends on its ability to withstand extremes. The brand's products have truly reached the highest, lowest, hottest, and coldest places on earth, with the ease with which the harshest environments on earth can be offered. Their success was largely due to an invention already conceived as early as 1926: the Oyster box.
This relatively simple innovation, one which has gone greatly unchanged for the last 90+ years, did more than anything before or since popularize the wristwatch. Beyond even that though, it has also formed the backbone for almost everything replica Rolex has created since - and solved one of the main issues facing any mechanical watchmaker: water resistance.
However, the usefulness of a model that could be worn on the arm was established during wartime, but the first examples proved relatively fragile. They were far more exposed to the elements than pocket watches and the earliest pieces allowed too much everyday detritus to enter the case and damage the caliber. 
Before the Oyster case's introduction, pocket watches were very much the only timepiece men used. Not so much for their practicality, but for the level of protection they afforded. Dirt, dust, and moisture have always been among the biggest enemies of watch movements, and so keeping a pocket watch tucked away inside the wearer's clothing when not in use helped safeguard its delicate internal components.
With the arrival of the Oyster, the brainchild of fake Rolex founder, Hans Wilsdorf, all of that was to change. Not only did the system of screwing the bezel, case back, and crown against the middle case form an impenetrable barrier against the usual debris, it was so useful that it also provided a level of waterproofing never seen before.
The basic formula has been built upon and updated over the subsequent generations until we reach today's situation where all but the Cellini range of dres' models in Rolex's catalog benefit from at least 100m of water resistance. It all perfectly encapsulates the company's original adage of building watches that are exceptionally handsome and stylish yet still resilient enough to be worn every day. Will any of these models ever be taken to 100m underwater? No, but a high waterproofing factor isn't just a sign of how deep the watch can dive, it's a testament to the robustness of the whole project. The idea is to show the skill behind the watch and reassure wearers that they are unlikely to encounter anything that seriously tests the watch's integrity.