This works fine and you can interact with slicer and obtain the result depending on the quantity you select.
And same can be done with the following, look at the ALL statement it will contain unique combination of Quantity and Net price and once the product is greater that 1000 only the values of these 2 columns would be applied to the filter context.
In case of full table all the columns of the sales would be applied to the filter context and that could be very expensive in case there are a lot of columns, and to be honest I don't think you would need every column of a table to get the result. And the number of rows applied to the filter context are huge too! (I say all the columns will be applied to the filter context, in this scenario the code is so simple that the engine know not to iterate all the columns, but in cases with nested measures or complex expressions, all the columns can be materialized)
Let's pay attention to the queries generated by these 2
without KEEPFILTERS query:
With KEEPFITLERS query:
By now you can see how many rows the SE engine has to bring back to get the desired result.
Let's say you are slicing trendy colors by brands.