Why do some dogs have fur the reaches the floor

 To be pretty.  Yep, humans are vain and many like fluffy dogs.  Most dogs are no longer used to do what they were originally bred to do and appearance has become more important than function.  Others like many toy breeds have long since started to be breed purely as companion lap dogs and their appearance reflects what was and now is considered most attractive and proper to those breeding them.  This isn't to say that some dogs with long coats didn't originally need them for practical reasons but what they have morphed into over years seems to be for looks, especially if you take some show dog lines and match them up to good working dog lines of the same breed.

Some dogs have "fur" and some have "hair" -- hair grows and needs to be cut.  Fur sheds and typically isn't cut unless the owner haves them groomed for some reason.

All scent hounds don't have ears that drag the ground - the ear flaps are probably (were probably) more for the purpose of keeping the ear closed to foreign objects and protection than 'stirring up dirt' (since the nose precedes the ears usually).  *** eared dogs (shepherds/herders and dogs bred for guarding/protection originally) move to cup sound. 

Some of that was original necessity -- some of it was simply a difference in breed and ancestry.

Many companion toy breeds were bred for their fur so that when they sat on the laps of their owners, they kept the owners hands and laps warm.  "Back in the day" before central heating and the like, people did various things to help stay warm.   From the little research I've done over the years, the more well to do people would have these small dogs to do just that, so the dogs were bred for tractability, low energy and long fur. 

Most breeds of today had some reason for being bred the way they were "way back when".