Whats the point of bobbins and do you need to have a spool cap?

I just bought a sewing machine and I have no clue how to use it.

It didn't come with bobbins and I was wondering whats the point of them if your using the thread roll from the top?

This is the machine I have and the tutorial

https://www.youtube.com/watch?v=n1tzZ_G3AmA

​

Also it didnt come with a spool cap. I was wondering if its needed? Thanks :)