I should also have said why not positive to battery clamp and negative to an earth point on the car.
I think that is what I actually said - on a negative earth car (as modern cars are), the positive charger lead can be connected to the positive terminal, or preferably if it has one, the positive terminal piggybacked off the main battery post usually found under a designated plastic flap with a big red
+, albeit sometimes only a matter of inches away from the battery terminal, and the negative charger lead to any solid earthed point of the car - away from the battery preferably.
Same applies to jump starting the car wherever possible.
Many people charge or jump start a car with the leads of the charger or jump leads / jump pack directly on the battery posts without any problems, but it is actually
advised to have the earth or negative leads connected away from the main battery terminal.
I always do it this way, and just think it is wise to do as the instructions advise.
I had a little information booklet that was stuck to the side of a Bosch battery in a plastic pouch that I left attached for the years the battery was in use. It eventually gave up the ghost, so I binned the booklet only a few days ago, but that clearly stated the points I have referred to about lead connection.