Land, and buildings, are worth a lot more on the open market with planning permission for development than without planning permission. Planning Gain was introduced so that public bodies could share in the increase in value of a plot of land when planning permission for development was awarded.
Some cynics considered this payment to be not that different to being a bribe paid to a council by a developer in return for planning permission being granted.
"Planning gain" payments were replaced by "Section 106" agreements, which were designed to define in more detail what the money paid by developers would be used for. The argument then was that the payment should be for remedial measures to the public realm in order to offset the impact of the new development.