The main Scotch distilleries will have a very specific style that they will always attempt to produce with each distillation. If the product does meet the 'house style' then they will bottle it under their own label, yet if it doesn't they will often sell it off to independent bottlers who will bottle it under their own label - this may or may not reference the distillery in which it comes from.
The most important function of independent bottlers is to go around and buy some of the best (and in 'house style') barrels from the distilleries. They will then age these particular barrels when they see fit (versus when the distillery needs to fill an order) which gives a much more complex and rich whisky. Generally speaking they won't add in caramel, water it down as much (you will often find many cask-strength whiskies) nor will they chill-filter it. All of those processes can compromise the quality. That along with the fact that these whiskies come from just the one barrel means that the resulting products are much rarer and harder to find. These are the whiskies to get for true whisky lovers!