Please don't get confused, ok let me explain you with a quote explaining Consideration which is "quod pro quo " which means something in return.
Now if you get something in return for supply made then you will be subject to GST but if nothing is returned for your supply then why should Tax be levied upon you. Have a look at this article which will help you understand in detail the provisions on Supply made Free of Consideration
Section 7 provides for Free of consideration.
With what I can understand that Section 15 is in respect of Supply made and there WAS consideration, but it's just that the consideration was not given in monetory terms thus valuation rules comes into place to compute a as per law Fair consideration or you can say Transaction value, I used the term Transaction but please understand that in case of Non-monetory consideration their might not be a transaction made as value toh pta hi nhi hogi in monetory terms ya disclose nhi ki hogi thus Valuation rules comes into place.