This is necessary if you want your customers to select a size, colour or other attribute of a product before placing an order. Each attribute can be the same price, but you can make separate prices for some attributes.

Here are the steps in WooCommerce to  firstly creating attributes, then adding attributes and variations with prices to products.

Creating Attributes:
Woocommerce: Products: Attributes:
Add Name of attribute (e.g. Size)
Configure Terms: (e.g. Medium, Large, X Large, etc)
See https://woocommerce.com/document/managing-product-taxonomies

1. Adding attributes to products:
   Edit Product: Product Data: Expand
   Change Product Data from Simple Product to Variable Product
   Click Attributes
   From Custom Product Attributes select Size
   Click Expand
   Select each one required or All ( Medium | Large | X Large etc.)
   Click Visible on the product page
   Click Used for variations
   Click Save Attributes
2. Prices on variations:
   Click Variations
   Click Expand (to show each attribute)
   Add Regular Price to each Variation (and Sale price if any)
   Save Changes
3. Update (Product)

See https://woocommerce.com/document/variable-product