1. It Will Humble You.
Every time you say 'If the Lord wills, I'll live, and then I'll...' or something like that, you'll be reminded of your own essential contingency. In other words, you're not necessary, and there's no reason to presume that you'll live. You'll be reminded by your own voice that you don't have the power to bring about what you plan any more than you have the power to determine if you'll keep living or not.2. It Will Give You Opportunities to Witness.
People will figure out pretty quickly that you're not normal if you're using this kind of language. Eventually, someone will ask why you're always mentioning God's will. When they do, you'll already be on the topics of the Creatorship and providence of God, and the fact that life is a mist and death is imminent--your life hangs on his will. Here is an open door for the gospel!3. It Will Give You Opportunities to be Ridiculed / Persecuted.
Christ himself pronounced blessing on all those who are reviled and persecuted for the sake of his name and for the sake of righteousness. Why would we expect the world to look at the future the same way we do, when all through James he has insisted that we have different perspectives and different kinds of wisdom? Being persecuted and reviled, then, becomes another opportunity to grow in the humility of obedience in submitting ourselves to God.4. It Will Change the Way You Think.
The way we speak is of utmost importance (as James always insists). As Lloyd-Jones has so famously said, we need to spend more time talking to ourselves than listening to ourselves. The way that we speak will effect ourselves more profoundly than anyone else. By being deliberate in the type of language that we use, we're training ourselves to think in biblical categories. When we change our words, it will change our thoughts, which will change our feelings, which changes us over time.5. It Will Reveal Idols.
If there is something that you're planning, or something that you desire for the future, and you're not willing to attach the thought, 'God willing' to it, then you're clinging to it too tightly. That fact alone reveals that you are looking to that future possibility to bring life, hope, joy, or peace--things we must find in God alone. Whatever you're not willing to give up for God (to hand over to his control to determine whether it will come to pass or not) that is an idol to you. It is a false god and needs to be put to death.6. It Will Force You to Think in Ethical Categories.
Sometimes we can think through future plans or situations without any reference to moral / ethical categories. Saying 'God willing' makes us ask, 'Would God be willing?' Once God--the standard of righteousness--is brought into the equation, we're forced to think in standards of righteousness. You can say 'I'm going to go to the party this Saturday night and not think anything of it. But you can't say 'God willing, I'll go to the party' without thinking about whether or not God would be willing for you to go. All of a sudden we're forced to reckon with God's thoughts on drunkenness, revelry, debauchery, etc., and that may inform our plans to go or not go.7. Silence Can be Sin.
James 4.17 is a verse that most of us have heard and know well, but have never applied to its context. James warns that to not take seriously the notion of God's will when considering our future is to sin. Simply saying these words is one way we can flee sin and pursue righteousness by acknowledging God, his providence, his will, his plan. Not saying it can be sin; but saying it can help safeguard our hearts from neglecting to consider God's will before our own.