What is the use of a second camera?

I have a Nikon D3300.

I'm an enthusiast not a Professional photographer.

What is the use of having a second camera?

What are the factors decide which camera to use. (possibly the second camera being a smaller camera like Fujifilm X20 or Fujifilm X-M1) Please recommend any other cameras that can be used as the second one ($500 max)

When I used a second dslr was covering a major event I volunteer for in my area here in Arizona. I would use one for video the other for stills. These Canon cameras I used were pretty similar in specs so I didn't have to constantly switch my video camera to stills and back again.