There is a giant split between MBTI and socionics perception of what Si even does. I think it is somewhere in between. Initially when I started, I thought Si was very underwhelming because all I could glean from the wiki and so-forth was that it recognized internal physical states and made people lazy. I didn't see what possible advantages it could have over Ni in the same slot, which recognized trends and flow of time. Later on I came to realize that it had a lot to do with the MBTI description of the function, despite it's perceived inaccuracies. Basically it compared and contrasted a huge number of images and sounds to look for differences and consistencies. This seemed much more useful. Then came the odd part.
This seemed to be the only way people used Si, particularily Si-, because delta STs are far more active then alpha SFs. Meaning, they put it to work in this way much more often. I remember reading about monks and how they would spend huge amounts of their life just focusing their mind on a single thing, and emptying or ridding of unnecessary thoughts. This enabled them to focus on things that really mattered to them. It should also be noted that most of them seemed to be Si egos.
In this information age, there are incredible amounts of information, and if Si is only used on gleaning information, it creates far too much information to cycle through efficiently. Si- I think generally focused on information. I think using Si- to dissolve unneeded sensory information is a very effective way to do it, considering the heavy load that Te+ applies. It is not so much of an issue with alpha SF, that tends to naturally seek down states.