Userscripts often have to parse the information from the results, such as looking for IP addresses, using client side parsing that is not always reliable. By adding appropriate classes it makes it easier for userscripts to find the relevant parts of the page and gives it hints as to what the information is. An example from checkuser-l is:

<bdi>220.101.83.145</bdi>

The BDI tag doesn't do much to help scripts or CSS recognize this as an IP. If it was more like:

<bdi class="ip-address">220.101.83.145</bdi>

that would simplify the job of add-on customization scripts.