Hi lally,
One scenario for using project sales orders would be if you purchase materials/goods and ship it to your project customer without the requirement to modify the materials/goods. To get the costs and revenue posted on the project you can use the ordinary sales order functionality and link it to a project. The only difference to an ordinary sales order is that you need to invoice the sales order through the project. Otherwise, the Revenue will not be recognized at the project level.
Hope it helps,
Ludwig