That the water level goes up and down due to the tide is an obscure reference? OK, there's this force called "gravity," where all objects are attra ted to each other in proportion to their size and distance. The gravitational effects of the sun and thee moon, combined with the earth's rotation cause a periodic rising and lowering of the oceans. Other randomly occurring factors also affect water level. So, on most seashores, the water level rises once or twice a day, an exceeds once or twice the day at measurable and predictable intervals.
If one has a house on the beach, then one has to know the general trend of water level. If due to erosion, tectonic shifts, whatever, the waters are rising, then it's important to know and take precautions (or find a sucker to buy the place).
But if you measure it at high tide and then at low, it will look like the water level has dropped very quickly, and vice versa. So to actually know what's going on, you have to measure the periodic changes and mathematically factor that out to know what's happening. The alternative is to average the levels or only compare high tide to high tide and low tide to low tide.
Same thing with Employment.... It always goes up in October and June, and always goes down in December and January. So very year it looks like employ goes down in January. But how much is the real trend and how much the normal, periodic, changes? How much is the tide? To account for it, you have to compare January nobly to January, or factor out the normal January change.
The adjustment s aren't made on a whim or any kind of decision. Here's the free non-copywrited software
https://www.census.gov/srd/www/x13as/
But please share your method: how can you tell how much of the month to month change is a trend, or simply seasonal change?