Why A diode conducting in the forward direction always drops some voltage across its junction, and that value depends on the semiconductor material and construction. Silicon diodes drop roughly 0.7 volts, germanium about 0.3 volts, Schottky types around 0.2 to 0.3 volts, and LEDs anywhere from about 1.5 to 3 volts depending on color. That is why a designer picks a low-drop Schottky for an efficient power supply or rectifier.
Watch out Peak inverse voltage is a completely separate rating, the maximum reverse voltage the diode can block without breaking down, and it has no relationship to the small forward drop. The forward drop definitely does affect the load, since it subtracts from the voltage that reaches the load.
Silicon 0.7 V, germanium 0.3 V, Schottky 0.2 V: the drop depends on the type of diode.