All products are loaded into in the Tweakwise feed together with their attributes by default. Sometimes it's impossible to put certain information in the feed. For example, when the shop that generates the feed misses certain data.
Let's say a webshop needs to use offline sales data to sort online products. The webshop does not possess this data. You can create an extra import that imports one specific attribute, for example, offline sales. Another example is when the attribute for stock needs to be updated more frequently than usual. The Tweakwise feed is a huge file, and so regenerating the entire feed just to update one attribute will have a huge impact and can take a long time. A standard attribute is a much smaller file that can be loaded more often, parallel to the complete Tweakwise feed.