They are important to applications to Ivies and such. They can be important to engineering programs below top 100.
If you get an unexpectedly high score, they can be important to 50-100 level schools. I helped someone get low 700s on the math SAT 2, who got in off the waiting list at about a #70 school. This person had good ECs, but wasn’t in enough AP classes and such to usually get accepted at that level of school. I am sure the subject test made the difference.