More Roon Questions

I’m going to ask a late question. This topic was talked about last week, but I need a little more info. I have this Lyngdorf MP-40 that is Roon ready. It’s headed my way from Denmark and will have it Friday. I’d like to go ahead and buy a Nucleus. From all the previous discussions last week I have settled on that route, along with Qubuz. My question is about these TB SSD cards. I am starting from scratch. I have no files or such to insert. I will be streaming from Qubuz, through the Nucleus to the Lyndorf. At least that’s how I understand it. I have never streamed before, I’m a shiny disc guy. Can’t I just stream through the Qubuz/Nucleus/Lyngdorf and call it a day? Lets say I want to listen to Dark Side of the Moon... Is it easier to grab the album from a pre downloaded file off of the SSD card than it is to just stream it off of Qubuz? What advantage is pre downloading it? Quicker ? Is it that much of a hassle to find it on Qubuz and getting it playing? I guess I just don’t get the process, but I know you guys will. Please explain in laymen’s terms ..
The reason I ask is because the standard Nucleus comes without any TB SSD (and what the heck does that stand for). The Cards are 400 dollars for a 2TB SSD, and I like to know what the 400 dollars extra gets me ..