The foam caused issues because it struck something on several occasions. That something, the orbiter, is not in the way anymore. The reason for having insulation on the tank is simply to keep the propellant as cold as possible. The Falcon 9's rocket engine runs on LOX and RP-1, and that second part is a refined version of what we put in cars and aeroplanes. They have less to keep cold compared to the needs of the SLS, as that uses both LOX and liquid hydrogen. Without the insulation, you would boil off more propellant and you would need to keep pumping it in at a high rate. They still need to keep replenishing up until tank pressurisation, but at a much lower rate. That's my understanding of the system.