Say the VAT rate is 20%. Now if you buy something for $100, install it and charge $100 to your customer, you get back $20 from the tax authority and pay them $20, so if billing cycles align no money actually flows to or from the tax authority. But if you add value, say by buying $100 in parts, assemble them and sell the assembly for $150, you get back $20 for parts purchased but collect $30 for the sale, creating a net flow of $10 to the tax authority.
If everything happens under the same tax authority this nuance doesn't matter, in total there's always a $30 tax on a $150 part, no matter how complex the supply chain. But if more countries are involved the difference matters: if a company in Poland makes parts worth $100 and a company in Germany assembles them and sells them in the German market for $150, that's $20 in taxes for Poland and $10 for Germany. With a sales tax that's only collected when selling to a consumer it would have been $30 for Germany and $0 for Poland.