To speed the adoption of renewable energy.
Solar panels are classed as permitted development and therefore do no require planning permission. A good thing.
However stand-alone solar panels are only considered permitted development if they are less than 4m2 and are more than 5m from your boundary. To install a stand-alone panel you would need a garden of at least 11m wide. The vast majority of properties in the UK do not have gardens of this size.
The irony is that a 30m3 shed more than 1m from your boundary is considered permitted development. Sheds are obviously a higher priority than renewable energy under the planning regulations.
The planning guidelines should be changed to classify stand-alone solar panels as permitted development, perhaps with a height restriction of 2m, and to classify solar fencing as permitted development.