Possibilities For Color

Tone color.
Let's say you're wanting to play a passage written for flute solo, and every 8-foot flute or tibia stop in the organ doesn't have precisely the tone color you have in mind.
You might try drawing an independent 16-foot flute or tibia, if the organ has one in the manuals, and play an octave higher on the keys.
If that doesn't work you can try drawing an independent 4-foot flute or tibia (there's often a big, burbley 4-foot flute supplied to the main manual), and play an octave lower on the keys.
Octave and sub-octave string stops, color reeds, chorus reeds, even diapason stops, depending upon their tone color, can also make for some very fine solos when used in their 8-foot registers.
Many other possibilities for color in organ playing are also there, through the use of stops in their purity, stop combination, the use of couplers, percussions, etc.
And that's all the wonder of music.