Why go to Schiphol for an intercontinental flight when you can just get it from Heathrow?
It depends on where you live. KLM operated flights from lots of UK regional airports to Amsterdam, in some cases airports that don't have connections to Heathrow. Fares change all the time, but often from places like MAN, GLA, EDI they are often cheaper than BA via LHR. For a while they charged a supplement at least on nett fare contracts for departures from Norwich because they worked out people would pay a bit more to travel from a local airport than travel to Heathrow.
However we are not living in normal times in the aviation industry. Example I was looking for flights to Johannesburg for November and from Heathrow via AMS KLM wanted £800 more in business than BA wanted in first. Also a friend was due to travel from Dublin via AMS to Rio on Saturday. The flight from Dublin to Amsterdam was delayed and he missed his connection. As it was a through ticket it was up to KLM at Amsterdam to rebook him on another Amsterdam - Rio flight and find him a hotel for the night. However after 4 hrs in a queue at Amsterdam he gave up and rebooked himself onto a later flight and found his own hotel. In the hope that KLM will refund him.