A pre-separator is rarely REQUIRED with a dust collector. Most of the time you can grind or polish with only a dust collector. However, the use of a pre-separator captures the majority of dust and debris prior to it reaching the dust collector. Therefore, it is an addition that greatly extends the life of your collector.
For large commercial jobs where the man hours expense is high, a pre-separator will quickly pay for itself by allowing 60 second breaks to empty. This means only stopping once an hour to empty the dust collector. If your projects allow a 5 minute stop to empty the dust collector bag every 15-20 minutes, the expense of a pre-separator can be saved.