First is the issue of safety, which many others have talked about here. Sometimes a bolt is the only way. Second is the fact that sometimes a bolt is actually

First is the issue of safety, which many others have talked about here. Sometimes a bolt is the only way. Second is the fact that sometimes a bolt is actually the lesser impact. Bolts last a pretty long time, and if a route is highly traveled, the bolt can help offload what would have been gradual damage to the rock, flora, and fauna. For example, if you have six climbing parties a day rappelling off a tree at the end of the climb, that tree is going to sustain damage over time. A bolt placed nearby can serve the same duty easier, safer, and with less impact to the tree. To draw on other recreation, have you noticed stairs on hiking trails? A popular trail can experience so much erosion on steep sections that something so blatantly "leaving a trace" as installing stairs is actually the lower impact choice.