"Why do you think it's important to make an appointment to talk to God every day?
There's no short answer to this one- there's a lot of reasons. A major one is to make sure that everything we do, is done for Him. It suggests prayer before bed, in which you can contemplate your day, see how what you did was for Him, and if it wasn't then it gives you a chance to make that right. You can right then apologize for it and be forgiven.
If you no longer pray daily, then God is no longer foremost in your thoughts, and you are more likely to offend him. It's the whole 'out of sight, out of mind' thing; I, at least, need reminders to keep me in the right frame of mind. Once you miss a day, it's easier to miss the next day, and slide away from it entirely.
It also helps deepen our relationship with God- I'm much closer friends with people I talk to every day, versus those I talk to weekly or monthly. It reminds me that He's there in my life, and I will more likely turn to him when I need him, as is only right and proper.