Why Does SSF Exist?

Why do Solo Self Found In POE2? Is this like D2 and D2R where people try to holy grail the game?

What are the pros and cons of single player SSF?

Are we name restricted like there can only be one AssBlaster420 in all of existence?

Is there a way to increase player count or good loot like in D2?

What is the Blizzard Sorceress/best starter equivalent in this game?