There are various reasons for excluding advertising destinations for your products on Google Shopping. You may wish to exclude destinations for your product if:
Your product has weak product margins (in order to avoid earning non-converting traffic in destinations involving the PPC model.)
You want your products to appear for free listings but not on the Google Shopping carousel.
You’ve signed a non-compete clause.
Your website is down for maintenance or a holiday. You will still be charged if a user clicks your ad, even if your website is temporarily inactive.
Whatever the reason, it’s entirely up to you and your Google Shopping strategy. Just make sure you’re making the right choice for each product by excluding destination or setting the availability as out of stock.