No software is perfect. There are ways to do things in a program you might not think that someone else might find some way in it to do other things with it. Bad things that may, in its worst kind, execute arbitrary code on you computer, that you might not want to be executed. Like computer viruses, or spyware. Also, some ways things are solved, they might imply an issue, which might enable someone to circumvent some features or disable some crucial features of the software.
As you all know, the ICQ service works in a way where your client software communicates with a server. And that server with the client software on lots of other's computers all over the world. And sometimes, there might even be an issue with the software on that particular server. Something wrong, that isn't good. And that needs to be fixed. However, that fix for the server's software might cause old client software for your computer to be unable to properly communicate with the server. You might become unable to see another user's status or even to send them a message. Just because your version of the client software is old and does not work like the server anymore.
 
And it's not like a bad thing, we all just need to install updates and upgrades from time to time in order to stay current. To maintain our security we need updates, and those updates sometimes mean a big change, but they are not a bad thing. The updates only want you good.