Manston
I certainly raised an eyebrow when KLM announced Manston but the answer is simply that being located in Kent (and where there is no competition or even routes full stop last time I checked) it is another region of the UK that they can feed through their AMS hub. I guess the aim is to tempt passengers who would usually use LHR long haul.
I believe PITR'S point is a valid one seeing as KLM codeshare on the BE SOU route. Basically they have the region covered. They do serve pretty much every reigional airport in the UK though so who knows!