Leave aside the fraud cases. There can be more than one author in many cases. A Research Scholar usually includes the name of the guide in publications. Including the name of the seniors as a token of gratitude is not uncommon. There are instances where anyone providing a little bit of help asks to include his or her name in the paper. I wonder why reputed nomenclatural experts of the world do not ask to put their names in lieu of the help provided! They could have thousands of publications! In molecular studies or collaborating investigations there can be multiple authors because everyone is contributing somethings towards the publication. I have attached a publication where there are 8 authors and 61 co-ordinating authors.
Single author paper is an indication and recognition that the writer is confident and independent as far as ideas flow, methodology/techniques and writing skills are concerned. But this typically comes with years of experience.
While in case of more than one author,there are many advantages -it will be a collaborative research work involving sharing of knowledge, skills and techniques , generation of new ideas/insights (related to the work),access to a wider network of contacts among the research community and also increases the possible visibility of the work being carried out.