In Odoo, when you create a purchase order (PO) and include prices for the products, those prices are saved in the product's vendor information under the Inventory tab. This feature is useful for keeping track of the prices that you have paid for products from different vendors.

However, if the prices for the products change, the new prices will not automatically overwrite the old prices in the vendor information. Instead, the new prices will be saved as a separate entry in the vendor information.

The reason for this is that Odoo keeps a historical record of all the prices that you have paid for products from different vendors, so you can refer back to them later if needed. This allows you to see how the prices have changed over time, and also helps with budgeting and forecasting.

To update the price of a product with the new one, you can go to the Product form view, click on the Vendor tab and edit the price of the vendor you want to update.

In addition, you can use the 'Price List' feature to set prices for different vendors for a specific product, this way you can set different prices for different vendors and also you can set the priority of the vendors, so when you confirm a PO, the system will take the most priority vendor to fill the price.

Also, you can use the 'Product Price History' to track the prices changes for the products.

In summary, Odoo does not overwrite the previous prices in the vendor information because it wants to keep a historical record of all the prices that have been paid for the products, so you can refer back to them later if needed.