The ES skimmers are essentially a more affordable yet still functional alternative to the higher end CS skimmers. Most people with tanks smaller than 50-75 gallons are not looking for a skimmer that is $300+. At the same time, many customers with these size tanks are not 100% happy with their Berlins or Turboflotors despite these skimmer being an excellent value because of their cheap price.
The ES Series was designed to fill this gap. Precision Marine addresses this need with the lower end Bullet-1, and AquaC does it with the Remora Pro, Urchin Pro, and the mid end EV120.