Why can we install CD and no longer need CD in order to go into software?

[kenghtt]

I just wanted to know why we are now able to install a software from a cd and no longer need that cd to be in the cd drive to run the software?

I remember 10 years ago we would always need the CD inside the computer in order to boot into the software.
Thankyou in advance

 

[aldan]

the only thing i can think of is that to save hdd space the program was on the dvd and not the hdd.hence the need for the cd.didnt run across it in any of my old computers tho.