105 is a heavy compound so you need 205 to take out the haze induced by the 105.
Sealant is for durable protection but some ppl think the shine is too plastic looking and add a carnauba wax topper. It's another layer of protection and imparts a warmer glow.
Remember, once you wipe them off, you are left with microns of a layer on the paint. That some of these sacrificial layers stay on as long as they do is amazing if you think about it...