They didn't exist when I went to school. Or at least, none of my teachers used them way back them. When I was in "teacher" college, they were becoming all rage. I took notes, but still didn't get the benefits. Even after I started teaching in the classroom, I didn't use them much initially. Thankfully, I've started to grow a little in the area and now, I've been using them with my daughter fairly regularly.
Basically, a graphic organizer is just a way of organizing information. The can be a big benefit to visual learners, but also a kinesthetic learner can benefit if you allow them to fill in the diagrams themselves. It provides a different way of seeing the information you are learning.
Generally, when I use them, I use them for science and history, but I did one for English yesterday. What's interesting is the one website that I usually get my organizers from gives them a 5th -8th grade level! I'm using them with my 6 year old, second grader. One thing, however, is that I don't let her know that she's doing anything different. To her, all second graders use a y-graph to review adverbs. Again, don't let "the man" tell you what you can or can't use with your student.
Here's a few examples of how I've used them.
This is a star graph. I had her write facts about mammals in each section.
Here is a Venn diagram comparing and contrasting amphibians and reptiles.
This is a tree diagram. We used words and pictures to explain feudalism.
Where to find graphic organizers? Well, to be honest, you can draw them up yourself, but I do save myself some time and effort and get mine from Enchanted Learning. If you're not a member, however, you can only access the thumbnail of all the organizers they have. I did find some for free here, here, and here.