This video published by Pfeiffer Vacuum shows how gas flows from the inlet of a blower to the multi-stage roots pump and out the exhaust.
The purpose of mounting a blower (the top section of this pump) onto a multi-lobe roughing pump is that it dramatically increases the pumping speed and extends the lower pressure capability of the pump. Typically the multi-lobe pump will have a speed of 100 cubic meters per hour. Adding the blower can boost the pump speed to 600 cmh to 1000 cmh, depending on the model
Source: A4 series data sheet. Pfeiffer Vacuum
These pumps can be robust enough to handle harsh processes. Like all harsh duty pumps, there is a nitrogen blow-by on the bearings to prevent process by-products from destroying them. Depending on the manufacturer, gas purges may be introduced into the various stages of the pump to help move corrosive material through. Manufactures will also run the pumps at elevated temperatures.
Pfeiffer Vacuum (Asslar, Germany) purchased the Adixen (Anecy, France) on 31 December 2010. This filled a gap that existed in Pfeiffer’s roughing pump portfolio. It also opened up markets for both companies.
Other companies that make multi-stage roots pumps are Edwards Vacuum, Kashiyama and Ebara.
You can learn more about roughing pumps in Understanding Modern Vacuum Technology.