I use infill width % adjustments to make sure my infill extrusions are coming out as solid as possible. If I want more or less strength, I adjust the Infill Percentage not the infill width percentage.
I don't mean to hint at anything. I just know what I use it for. If I watch my prints and see that the infill is coming out fuzzy and not consistent I will increase the extrusion width or decrease the speed on my next print.
Infill width will increase the strength of a part to a limited extent, creating thicker and more solid walls at low infill percentages. Using an infill pattern like grid will help more than increased width alone would, and increasing the infill percentage helps the most. At 100% infill, I doubt infill width makes any significant difference in strength at all.