Use of reducer

It depends upon whether the line is needed to carry out liquid drain functions in near-horizontal lines, in which case the eccentric reducer will prove a valuable asset; think about it, Grasshopper....

You generally use a eccentric reducer for a couple of reasons.

Firstly if you need to wash the lines (like in a dairy application) and if you use a concentric reducer then liquid will sit in the line and not drain properly.

Secondly it's also easier for the guys to build the pipe supports since the base line of all the pipes are the same. So you can cut XX number of stubs regardless of the size of the lines.