As someone whose job it is to write CSS all day, I couldn’t tell you. I think it’s to accommodate the previous/next button icons, but there are much better ways to do that.
You’ll have to go into the main plugin file (logo-slider.php), scroll down to line 650, and edit the CSS from there. There are a bunch of !important tags, which cause the attributes to be un-overrideable, so make sure you deal with them. Additionally, there are several media queries which contain redundant CSS (rather than only the attributes which should behave differently), so if you want to change something, make sure you get it for each media query.
I grabbed this plugin earlier today because the demo looked great and I needed to drop some linked logos into the company website, but ended up gutting the CSS and cutting the slider Javascript (didn’t need the slider functionality) to make it work. Also, the included Javascript adds a bunch of inline styling to each list item, which thwarted any attempt to modify positioning of the items, so be careful if you try to edit stuff like display and positioning.
Oh, also, I’d suggest adding some custom CSS to the .logo-img class to limit how big the logos can be – one logo I added was a bit on the big side and showed up full size. I guess the plugin assumes you’ve already sized and cropped your logos.
