That's a dangerous question to ask a philosopher, because I could go on at great length.
The concept of conflict of interest basically has to do with the fact that in modern life we often have people making important decisions on our behalf or advising us on important decisions when we don't necessarily have either the proximity or the knowledge to monitor them, and so we need to be able to trust them, yet all decision-makers, including professionals and political leaders, have rich and complex lives that bring other factors to bear, so there's every chance in all kinds of situations that when you're entrusted with making a good decision on behalf of your employer, on behalf of the public or on behalf of your institution, other factors can intervene. What we want to do in those cases is try to figure out, given the necessity of this role, how we mitigate the challenges that might occur, including things like conflict of interest.