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It has been mentioned to me i should separate columns that are used often from columns that are not. Why?
Is it because a table with less columns is better for the cache? Is there an optimization for caching all columns rather than selected? Is this for tables frequently read? Should I separate freq write columns into their own table?
When you update a row, many databases will make a copy of the entire row. Depending on how long the copies of the row persist (which depends on many factors) you may end up with more garbage in the database than if you had separated the row into two tables. This garbage will make the database larger, and therefore may make queries slower.
Whether you want to prematurely optimize by denormalizing (which you would have needed to do to get rows that could benefit from being split) is something you should consider in any case.