As the Emperor of Mankind was preparing the Great Crusade, he realized that while the Legiones Astartes were excellent as the "spear tip", or vanguard troops, a spear tip still needed a wooden staff to support it. In other words, he needed regular humans to support the Legions. And while Terra alone had billions, the galaxy is a big place, and they couldn't hold every world with just Terran armies. To get around this, the Emperor imposed the Imperial Tithe, a set of payments that a compliant world must pay to the Adeptus Terra for continued protection. For this, an Imperial Commander, or Planetary Governor, must give the Imperium of Man not monetary taxes, but the following:
- They must pay a tithe of manpower in regiments for the Imperial Army, and later the Imperial Guard.
- They must provide goods for warfare, such as food, weapons, vehicles, or minerals to make any of the above.
- They must provide the league of Black Ships with all the unsanctioned-psykers that the planet has managed to capture.
Space Marine homeworlds and Adeptus Mechanicus Forge Worlds are exempt from the regular Imperial Tithe. Instead, Space Marine Chapters must provide a tithe of gene-seed to the Adeptus Terra, which is turned over to the Adeptus Mechanicus for testing to make sure that the Chapter's gene-seed is stable, or be used to make a new Founding of Space Marine Chapters. The Adeptus Mechanicus is free of the manpower and psyker tithes, but must pay with machinery used in warfare (and the occasional Enginseer to keep everything running).
Monetary taxes are officially separate from the Imperial Tithe, and weren't added until much later when the Emperor retired to Terra and Malcador the Sigillite took over the Council of Terra. Horus Lupercal, Primarch of the Luna Wolves, had concerns that the taxes could end up raising rebellions on planets. In the following ten thousand years, payment of the Imperial Tithe has resulted in planets declaring rebellion, but these in turn are crushed with extreme prejudice.
This said, the tithe system isn't utterly one-sided. Despite the high burden of an "Exactis Extremis" designation (the highest conventional tithe level), many worlds would prefer to keep it as this means that the Imperium will in turn invest resources into the planet and ensure that it is protected. Having their status downgraded is rarely good and if it drops low enough, the Imperium functionally cuts the planet loose and forgets about it - why bother investing or protecting something that isn't useful. Then again, one of the most common causes of rebellion in the Imperium is the tithe rating of a world being jacked up to levels that would require conscripting every single viable citizen, enslaving the entire population for labour and having children work the moment they can walk. Occasionally all 3 at once, because the Administratum rarely sees these things as mutually exclusive or as a cause for concern.
Real World Parallels
The most direct parallel from the Imperial Tithe and Real World history would be the operation of economies in the bronze age. In that system there was a class of scribes and bureaucrats employed by the state which took and kept track of what was produced and allocated it to various people so it could be used to achieve various tasks. Peasants farmed, provided a percentage of their crop to the state who used it feed artisans who made tools, cloth, beer and similar, which was taken by the government and provided to various peasants, who'd also spend some time working in mines or clay pits and working on large scale agricultural projects. Raising Troops worked along a similar line. How a group of peasants was to get the job done was broadly a matter for them to sort things out, as long as they met or exceeded their quotas was all that the scribes in the palace cared about.