As pbuilder mentioned, they are not actual bath salts meant for putting in a bath - real bath salts are various salts such as epsom salts or dead sea salts, often with fragrance. Whereas the drug "bath salts" are just called that because the drugs in them are not legal for human consumption, so if they pretend it is a bath salt they can get away with selling them - although many of the drugs in these "bath salts" are now being outlawed in some countries. Also "bath salts" can contain a number of different drugs, not just MDPV. It can be dangerous because you don't always know what you are getting, plus these drugs are new and we don't know about all the risks yet.
As to why people would want to do it, the same reason most people do any drug, because they are hoping for a pleasurable effect. Not really sure what else can be said in this thread, if you'd like more info on MDPV you can do a search for it.