What is the primary purpose of a screen grid in a vacuum tube?

A vacuum tube with just a cathode, control grid, and plate is called a triode. One of the main limitations of a triode is the internal capacitance between the control grid and the plate, which can cause unwanted oscillation at higher frequencies. To solve this, a second grid, called the screen grid, is inserted between the control grid and the plate. This creates a tetrode. The screen grid acts as an electrostatic shield, effectively decoupling the control grid from the plate. By drastically reducing this grid-to-plate capacitance, the tube can operate stably at much higher radio frequencies without requiring complex neutralization circuits. Distractors like increasing efficiency or changing plate resistance are not the primary purposes of this specific grid.