Consider a situation: You accidentally move your mouse pointer to over the Ubuntu launcher while browsing a website in a web browser. And then when you try scrolling the webpage using mouse, you instead end up switching to the app over whose icon the mouse pointer is (assuming that the app is open).
Yes, that happens - try it out.
Want to disable this feature? This can be done using the following command (tested in Ubuntu 16.04):
gsettings set org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/ scroll-inactive-icons false
To re-enable the feature, just replace 'false' with 'true':
gsettings set org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/ scroll-inactive-icons true
Alternatively, you can make this change through the dconf-editor as well - just head to org->compiz->profiles->unity->plugins->unityshell, and then tweak the value of scroll-inactive-icons option.