[@WanderingDragon]
1- You can choose your own servant. As mentioned in the Gatcha, it's totally optional. The main appeal is for people who don't know who or what they'd want to play, so they're basically asking for a random idea. 
2- It is optional to do so. Yes. There are only four masters, and eight servants. You can make a servant but not a master, or a master but not a servant. But you're only allowed to make one of each. So you can't have two servants, or two masters.
3- You don't need to know too much about the franchise to get involved. General rule is "Bad guys are messing with history, so mages summon famous people to fight the bad guys."

[@SSW]
It's an optional way to get a random servant idea tossed at you. We have Twenty-One random ideas from various mythologies/histories. If you decide you don't like the idea presented, you can request a reroll or just make your own. You are not obligated to use one. It's just for people who might not know what they want to make. 

Say for example you want to roll an idea. We'll spin a wheel, and then send you the name, possible classes, and a brief summary of a character. 

Ex: 
[b]Pocahontas (Rider/Caster)[/b] - Princess of the Powhatan's chieftain. She married an Englishman, and acted as a speaker on behalf of her people to protect their culture from the English. Her story has been romanticized over the years, and she is a subject of art, literature, and film. Her true name is supposedly Matoax, which was carefully concealed from the British because knowing her true name would enable them to cast harm upon her. 

Giving you a basis to create a character sheet around the identity, and a little blurb for a possible noble phantasm.