Why do we use Natural House to cast a general horoscope?

You can create any chart as long as the date of birth is set to the 1st of whichever month you are interested in generating a horoscope for.  One reason to use Natural Houses is to be independent of location.  Our astrologer and founder, Henry Seltzer, recommends you watch this new moon video he just made.  He thinks there's a summary in there that you will find helpful. 

When you cast a chart for the first of the month and set a sign under Natural Houses, it is independent of location and will read for whatever sign you choose.  For example, the Natural Sign chart for this month's Full Moon with, let's say, Leo chosen generates a chart that makes sense for a Leo - the basis of our monthly forecast material by Sun sign.  

The chart with Natural House set to Aries rising is useful when thinking of the chart for any sign, any locale. When we say Solar chart (which is what we cast to get our horoscopes) we mean for any stated birth time or sunrise but with the Sun sign leading in Natural House, e.g. Aries chosen.