As you undoubtedly know pumps are designed to develop head so the pump can be sized to deal with any losses you design on the discharge. The suction however is another story. The suction of a pump generally has less than one atmosphere of pressure available to inject the fluid into the pump properly and you have virtually no control of that. If you get the design of the suction wrong the pump will generally either stall or cavitate. It you get the design of the discharge wrong generally all that happens is that you loose or gain a bit of flow. The suction of the pump is by far the most delicate part of the installation and should be given at least half of your attention to ensure that you get it right.
You generally can not design more pressure to feed the pump so in most cases the best way to increase the available pressure on the pump suction is to cut down pressure loss in the suction line and that is most easily achieved by installing a suction with a larger diameter than the nozzle on the pump. Hence the need for the reducer at the pump suction.
Reducers have one unfortunate characteristic and that is that they have a built in high point or a built in low point or both depending on what you install. These high points can help in the accumulation of either bubbles of gas or slugs of liquid depending what you are pumping. Say you are pumping water, air tends to collect at the top of a reducer as a bubble. If that bubble finally gets sucked into the pump it could cause the pump to stall. If you are pumping a hydrocarbon like diesel, water could collect at the bottom of the reducer, and when it gets sucked into the pump it gives the impeller a jolt due to its higher density. It could also form an emulsion as it passes through the pump, and there could be any number of process, corrosion or other reasons why you would not want this slug to develop.
It is therefore common practice to install eccentric reducers and on liquid lines that carry a little gas the flat side of the reducer is put on the top. That ensures that any bubbles that come along the top pass immediately through the pump and do not accumulate. When liquids are pumped that carry a small fraction of a heavier and immissible liquid the practice is to put in an eccentric reducer with the flat side on the bottom. That ensures that when drops of the heavier liquid come along the bottom of the line they too pass immediately and unhindered through the pump.
If you have a clean liquid that does not carry any gases or liquids, it would always be better to put in a concentric reducer because it will ensure that the liquid hits the pump suction with the most even flow pattern.