What does oranges and flowers have to do with painting portraits? They provide the experience of shapes and techniques...in other words no experience is wasted. 

Exercises found in art classes endeavor to break the student's notion they know an object. We all have preconceived, 'maps', of faces. In deed, the smiley face is rudimentary foundation a portrait. Ovals  improve the map, but falls short of what is in front of the artist. It would be more realistic to treat the face as a mountain; with all its ridges, valleys, and mountain peak.

The lesson from painting unfamiliar subjects is to leave behind  preconceived notion of our limited grasp of the real world. 

Things I have learned from unfamiliar subjects:

• Leaves: taught me about cutting into the shape with the background colors. 

• White iris: white is not white, but, pale shadows and subtle tints of local color.

• Tangerine: taught me about subtle reflections and translucent skin and meat of the fruit. 

• Horse hide: the subtle transitions of cool to warm colors.

The list could go on; once again: no experience is wasted.

People might say I know how to paint flowers or horses. But, I really don't.  My ability to observe, to apply, and study shape and form to make a reasonable facsimile is key. However, it does leave me with a larger library of shapes, tones, shadows and color to be applied to the next portrait.

Keep observing and more you know the more you realize you don't know anything; it's all an illusion of perception. However, we can still update our perception of the world around us to better understand the our illusion of the world and those around us.