In English when defining a noun, it is typical to use indefinite article. For example an apple or a bicycle. Because we're talking about apples and bicycles in general, not meaning any in particular. In fact it would not make any sense if I see noun definition accompanied with definite article.
But in French I see more often noun definitions starting with le/la. In the book Oxford take off in French in particular, every new noun is introduced with definite article which doesn't make any sense to me.
Thank you all for your answers! You helped me to solve my doubts. Below, I will outline the most important answers that I completely agree with.
My best guess would be that it make it a lot easier to actually hear the gender: "un" and "une" sound frighteningly close, especially if you have a non-French accent and tend to pronounce the 'n' at the end of words...
Going from le / la to les is smoother than going from un / une to des. The latter requires knowledge of the usage of de, and to a novice that would be a leap.
And rephrasing @subtenante (however it doesn't directly answers the question):
Indefinite article is better since it doesn't cause the elision when followed by a word that begins with a vowel or silent 'h'.