I use Signal because I think it protects my SMS messages from:
a) being harvested and read by other apps on my phone
b) being read by someone who unlocks my phone
c) being passively intercepted and stored by carriers and their snoopy employees
d) opposition researchers or private investigators targeting my friends, acquaintances, and business associates.
For anything targeted and state level, all bets are off anyway, so it's not a solution for people who have that problem. What am I missing?