Taking a look at the picture above it is very distinguishable the difference in Infill percentage for the same part.
But Why would you pick one or the other?
It is a balance between cost and strength, depending of what is needed to achieve.
On lower Infill percentage the amount of material that is used to print the part is also lower, which in turn will also print faster since there is less area to print, and for this reason when the infill is increase, it also increase the cost of material and the printing time of the part.
The strength of a design is directly related to infill percentage. A part with 50% infill compared to 25% is typically 25% stronger while a shift from 50% to 75% increases part strength by around 10%.
Increasing the infill percentage will result in a stronger part and it uses more material, it also take more time to print therefore it will increase the cost of it.
The same happens the other way, less infill, less material, less time equals more cost efective print.
Picking the right amount of infill material will depend on the final use of the part. If you need screws through it, will need a better anchor inside is a must, it will need more material (About 50% to 70%). The same way if it have snap-fit parts, It will need above 70%-80%.