OP - You are confusing two different things. You keep referring to "Economy". That is the "Main Cabin". You are traveling on a fare code known as "Basic Economy". It is simply a cheap fare basis which will seat you in the Economy cabin.
As many have noted, Basic Economy comes with many restrictions. One of them is the inability to obtain an advance seat assignment. Whether saving $20-30 is worth it to you is a decision only you can make.