There are two main benefits to providing such facilities for drug users. First, the rooms would presumably provide clean needles for the users. This would greatly decrease their risk of contracting HIV or other diseases that come from sharing needles. Second, the room would provide a safer place for the users so they would be less likely to harm anyone or to be harmed while using.
However, there are many negatives about doing this. Let us look at two of the most important of these. First, it would be like condoning drug use. We do not want people to use drugs and so we should not seem as if we want to make it easier for them to do so. Second, any place that was used as a drug injecting room would cause major problems for its neighbors. Businesses would surely see customers scared away by a nearby drug injecting room, for example.