Suppose your doctor tells you they have found colon cancer. You have to make a decision whether to have surgery. If you thought there was no such thing as free will because it is causality all the way back, you might say, well, it’s futile, I cannot make a free decision anyhow, I will just sit here and wait. That would be foolish.
If you are crippled by the thought that it is causality all the way back, you have essentially made a decision to make no decisions. That is very unwise. If by thinking that free will is an illusion you believe that it does not matter whether you acquire good habits or bad, hold false beliefs or true, or whether your evaluation of the consequences of an option is accurate or not, then you are highly likely to make a right mess of your life.