Think of how the legs of a folding ironing table, table, or chair work: the scissor-like legs allow the table surface to move up and down, while remaining parallel to the floor. The scissors allow vertical movement without toppling.
In a regular keyboard, the vertical motion is regulated by a column of plastic that holds the keycap. In a very flat, laptop keyboard, it’s not possible to have a column that’s tall enough to hold the key vertical. So, instead, the plastic scissors keep the keycap level.