Why is iodine used when observing onion cells?

Answer

Iodine was used to stain onion epidermal cells so you could see the cells more clearly and examine them. Naturally, the cell parts are difficult to see because they are clear.

You put iodine on onion cells in order to make parts of the cell which were translucent, more visible. This is because iodine stains starch present in the onion cell.