Why do some programs require CDs to work/open/play, while other programs install everything to hard drive?

It is not really meant to save space. Some programs use copy protection while other programs don't. It's meant to stop people from using the software if they don't have a physical CD.

They do that so you will need the CD, so you don't let someone else put in on their computer for free. There's usually no extra information on the CD you put in. They just do that to try to prevent file sharing (aka piracy)

You can't, its the software that decides if you need the CD or not. Having to put a CD in is just a basic way of (trying to) prevent piracy anyway, it probably doesn't save much space on a hard drive. If you're having problems with hard drive space then you may as well buy a bigger hard drive, or just a new external one.

The reason is copy protection. You could download a program for bittorrent, but you cant use it because you have to put your cd in to use it. You can't change this, and you can't make programs run from Disk easily, Sorry. But you could look into portableapps (google it), you can put them on a cd and use them off the CD

That's the way they are designed and you cannot make any changes.

Well the reason for this is because certain programs read files from an ISO (or disc image). To get a collection of files to a disc (within reasonable size) I use a program called PowerISO. But I mostly use the program the other way around where I mount my own made ISO images to invisible drives. I prefer to consume Hard Drive space because you can always get external hard drives. CD's can get scratched and lost, so its not always best. Plus there is always a potential that your laser might not work after some time or in some cases you consume more CPU in running your CD Drive rather than the traditional old hard drive!