Now, in the quantum moment before the closure, when all become one. One moment left. One point of space and time. I know who you are. You are destiny.
If I'm getting this right, you're asking why Halo Custom Edition asks you for the CD in order to play. Yes, you need the CD inside your CD Drive for Custom Edition to run, even if a couple of sites have it for you to download for free. But I believe update 1.08 removed Halo's dependance on the CD in the Drive in order to run, so I would guess you haven't updated Custom Edition (or either in any case) yet.