Why use a Schottky diode instead of a normal diode on a solar panel?
No, the voltage drop is not about the same.  Schottky diodes have half the voltage drop compared to otherwise equivalent full silicon diodes.  And, that's the reason.

When a solar cell is dark, it is simply a silicon diode.  The diode polarity is opposite the current flow when illuminated.  For a series string of solar cells, a single cell in the dark blocks the current regardless of what the other cells are doing.  Even worse, with enough other cells in series, the cell in the dark can be reverse biased to the point of destruction.  A Schottky diode in parallel allows the current from other cells to bypass a dark cell, and limits the reverse bias voltage to a safe level.