Simple reason being color contrast
It simply helps differentiate between the areas of the character's body see how the tan on the arms makes it stick out from the body and legs
The boom design, just looks like one big blue blob with no distinction between one part of the body ends and starts
Superman with his lack of underwear had the same issue, no color contrast
If they wanted a superman design with no underwear then you need to redesign the entire character so that one piece of color contrast is no needed like they did with batman
Look at post redesign batman, the more intense shading with the more prominent contrast of the suit's own design, With the grey and black kinda rendered the blue underwear mute
versus the pre redesign, Where the blue did not contrast well with the grey so they had him wear blue underwear so it stands out the upper body from the lower body
It simply helps differentiate between the areas of the character's body see how the tan on the arms makes it stick out from the body and legs
The boom design, just looks like one big blue blob with no distinction between one part of the body ends and starts
Superman with his lack of underwear had the same issue, no color contrast
If they wanted a superman design with no underwear then you need to redesign the entire character so that one piece of color contrast is no needed like they did with batman
Look at post redesign batman, the more intense shading with the more prominent contrast of the suit's own design, With the grey and black kinda rendered the blue underwear mute
versus the pre redesign, Where the blue did not contrast well with the grey so they had him wear blue underwear so it stands out the upper body from the lower body