All the functions are motivations, Si included. None of them are really "for" anything - they are a way of describing how different motivations interact and conflict with each other. They are not skill sets or even cognitive abilities like remembering information or organizing things.
Si is concerned with certainty of knowledge. It is a drive to make sure that you know for certain, or at least to get as close to that state as possible. Potentially, anything that feeds that need for certainty could be an expression of Si, where it is making sure that all of ones references are good, keeping a well organized to do list or just deciding that something isn't worth the risk invovled. It is associated with planning ahead and needing to know what will work in any givgen situation.
Compare this to Ne, which is the urge to explore new experiences because of what they represent. It is spontanious, needing little reason to do a given thing apart from "it might be interesting". THe care and attention of Si clashes with the urges of Ne, which is why these two functions alienate each other. Having one in the primnary position forces the other into the inferior.