The Power of water
Water is one of the most powerful and common substances on Earth, simply because it is such a necessity. Think about all the things you use water for everyday: we drink it, cook with it, bath in it, swim in it, eat food grown/fed with water. Water is contained in the air, oceans, lakes, rivers, streams, watersheds, soil, glaciers, snow, rain, puddles on the ground.
We know that different locations around the globe have different overall temperatures and precipitation patterns. I can tell you this just from personal travel experience, say compare that time in Puerto Viejo Costa Rica where it rained nonstop for eight days, to the point where everything I owned was somewhere between damp and soaked, while the farmer I was working for cheerfully dawned waterproof overalls and coat and went about his work. While, in Knysna South Africa it was hot and dry all week and then suddenly rain an apocalyptic amount in three hours, yet these rains are enough to maintain a lush landscape.
Like temperature plants evolve to live with certain water requirements. Take for example a cactus: Cacti have evolved to store massive amounts of water within their bodies, and photosynthesize at night to avoid greater water loss (how cool is that), in other words they have adapted to a low water environment. Plants in environments where incoming water is more consistent simply don't have to have such mechanisms.
Precipitation in Darjeeling
The Darjeeling tea-growing region greatly effected by the summer monsoon climate pattern of an extremely wet summer, and a significantly dry winter. The monsoons are a complex climate pattern created by interacting factors, but very generally speaking this pattern is caused by temperature (and thus pressure) differences between the continental landmass and the Indian Ocean. In the summer months with greater amount of incoming sun the continent heats up quickly, causing the warm air over the continent to rise. At this point the cooler and wet air over the ocean rushes in to fill the "gap" left by the rising warm air, this leads to the massive amounts of rain in Southeast Asia in the summer.
I found a similar explanation in an article posted by MIT: "During the summer, the huge landmass of Asia heats up like a brick in the sun; hot air rises over the continent, and cool ocean air, filled with moisture, flows in to replace it. During the winter, the air over the Indian Ocean is warmer than the air over land; the ocean air rises, drawing cool dry air from Asia out to sea."
In Darjeeling, the months with the greatest rainfall are June through September when the bulk of the annual rainfall occurs. According to the Indian Meteorological Department, based on a 99 year average, the mean maximum precipitation occurs in Darjeeling in July with 781mm of rain falling over the area. Compare this to the mean monthly minimum, based on that same 99 year period, in December where only 7mm of precipitation falls, and you start to see the power of the monsoons especially considering the total average annual rainfall is almost 3000mm per year in Darjeeling. But the monsoonal rains in Darjeeling do more than feed the agricultural sector: the bulk of articles I found examined the effects of heavy rainfall on land stability in Himalayan foothills, citing the seasonal rains for the large amount of landslides on unstable slopes.
Tea plants generally need a lot of water, especially those planted on slopes where greater amounts of runoff might occur. In a 1972 paper author M.K.V Carr found: "The minimum annual rainfall generally considered necessary for the successful cultivation of tea is between 1150 and 1400 mm, but it is the distribution of this rain over the year which is important, specially relative to potential evaporation rates.” Carr continues: "For each unit of rainfall received, the early [November and December] rain was found to be twice as productive as the main rain [January - May], whereas the dry season rain had a depressing effect on yield."
It's also important to note that not all water that hits the surface is going to be available to plants - some will runoff and never infiltrate the soil, while high evaporation rates will deprive the soil of water, especially in the upper layers. However, overall Darjeeling seems to be a location that lends itself to a bountiful summer season for tea gardens, despite the low precipitation and temperatures in the winter months that prevent the region from having year round harvests.