Question
Dear Tom,

I do know that plant would convert glucose into starch for storage. But I would like to know the rationale behind.

Is it because of the ease of storage or the ease of energy release? Anything related to its structure?

Thanks!

Yours sincerely,
Raymond

Answer
Raymond,
Thank you for your question.  I am an horticulturalist, not a botanist.  My expertise is in the area of growing and cultivating plants not necessarily their biology, physiology, etc.  However, I will try to answer this question as best I can from what knowledge I have of basic biology, physiology and botany.

Some plants do convert glucose to starch.  Glucose is created through the process of photosynthesis that is the plant using the energy of the sun to create sugars which are subsequently stored in the roots of the plant.  Some plants such as rhizomes, corms, bulbs, tubers, etc.
convert the glucose into starches.  Examples are, rhizomes (iris or canna), corms (gladiolus), bulbs (tulips, onions), tubers (potatoes, dahlias).  The reason for this to my knowledge is that starches seem more prevalent in tubers and bulbs.  The reason for this seems to be that it is a quick store and then a quick release.  Plants that store a lot of starches in their rhizomes, bulbs, tubers, corms etc., tend to be vigorous plants with large blooms and many of these plants will bloom continuously until their energy runs out.  They will then develop large leaves or will already have produced large strap like or flag like leaves which gather sunlight for production of sugars and starches for next year's growth.  

Well, that's the best I can do.  I'm not a botanist.  If you want a really professional answer, my advice to you would be to contact your state's agricultural college.  If you would like post a follow up and tell me what your state is and I will respond with a contact for you.  

I hope this helps.
Tom