Yeah its an application for a Lawful Development certificate, basically they are asking the Council to confirm that the use of the site is lawful and thus cannot be enforced against, in this case it appears because the site has grown somewhat beyond its original PP and there seems to be no PP for the extension areas.
This tends to depend on the time elapsed since things happened, where there has been a breach of planning control involving a material change of use or breach of a condition no enforcement action may be taken after the end of a period of 10 years beginning with the date of the breach. For operational development, such as the erection of buildings, the time period for immunity from enforcement action is shorter, this being within 4 years from the breach of planning control.
As for why, frankly I have no idea there are loads of potential reasons, establishing the approved use could be the initial phase of wider scheme to change the use, equally it could merely be that they have been threatened with enforcement action and want clarification, equally they could be wanting to sell the site and want its use legally established - until this is cleared up and any subsequent applications are (or are not) submitted no idea!
This advice is provided on an informal basis and all that and where do I send the invoice?