Everyone, who´ve ever tried to solder without a flux and then with a flux, probably has seen a big difference in solder spreading and in a resulting joint quality. This effect is very visible on a PCB with bare copper tracks without any surface treatment (for example at DIY production of prototypes) but there´s also a significant difference at boards with a surface treated by tin (HAL) or by gold (Flash, chemical gold,..).
Especially at soldering of SMT components it´s often even impossible to solder by a method “solder wire in one hand, soldering tool in another”, because we usually need one hand to hold a component in tweezers and moreover – often even a 0.5mm solder wire is too thick to precisely dose a solder directly to a small component in for example an 0402 package. That´s why we usually solder SMT components by a method of taking a little bit of solder on a soldering tip and then applying to a component.
FLUX SK 10 (originally called also a LOTLACK SK10) is based on the natural rosin (colophony) and after application it creates a thin layer. This layer makes soldering significantly easier and at the same time it protects from corrosion.