What are the implications of using important in CSS

Question

I've been working on a website for a while, and I frequently have to use it when trying to modify something. important, for example:

div.myDiv { 
    width: 400px !important;
}

to make it appear how you would expect. Is this a bad habit? Or the case! Is it allowed to use the important command? Is there any potential for future harm from this?