They are most of all authentic,students are learning through a content they have previously negotiated ,so theiur interest arises exponentially . they involve students creativity because they have to decide on the way they are going to present it,negotiate with classmates who are working with them,which involves a lot of learning in working collaboratively and specially critical thinking,when they are working on projects,they have to prioritize,discard,explain,all activities which involve criticakl thinking.I also believe PBL is a very good tool for Differentiated Instruction ,because it gives each student a chance to do their best,and time to finish and present when they are ready.
It is the same with webquests ,this year I implemented several ones but there was one specially that worked very well the topic was "How Media Influence Famous Couples ".I gave students several webpages to look at,and they prepeared excellent Power Point Presentations to class ,they were highly motivated with it,and as a result their behavior was better than usual,they really worked harder than ever.