That's much more clear; now I believe I understand your question. You're asking then, if multiple software packages are listed for installation, why does apt-get download all of them before installing?
Assuming I have correctly interpreted your question this time, here is the answer: the current packaging system first requires that its package cache is updated (ie. apt-get update) and from there, it can determine from the package cache which packages need to be installed, and which dependent packages also need to be installed. The next step after that is to download all of the packages. Recent tools can parallelize some of that downloading; for instance, the recently announced Debian redirector mirror, located at http://http.debian.net/debian, can utilize multiple sites, concurrently downloading 2, 3, sometimes more packages simultaneously, so that helps out a LOT. But until all software is downloaded, the system as currently designed cannot procede. After that, the packages are expanded; a .deb package consists of a file list containing the target files, an archive that needs to be uncompressed and expanded, and that is the next step. Only after that is complete are packages ready to be set up, and that is the final phase of the package installation.
Here is a recent example; I was installing the linux headers for an antiX Linux system, after just installing a matching kernel:
By the way, the Debian mirrors HTTP redirector is explained at http://http.debian.net/; do give it a try!
Get:1 http://www.daveserver.info/antiX/debs/ testing/main linux-headers-3.4.4-antix.1-486-smp i386 3.4.4-antix.1-486-smp-10.00.Custom [7,813 kB]
Fetched 7,813 kB in 30s (259 kB/s)
Selecting previously unselected package linux-headers-3.4.4-antix.1-486-smp.
(Reading database ... 124727 files and directories currently installed.)
Unpacking linux-headers-3.4.4-antix.1-486-smp (from .../linux-headers-3.4.4-antix.1-486-smp_3.4.4-antix.1-486-smp-10.00.Custom_i386.deb) ...
Setting up linux-headers-3.4.4-antix.1-486-smp (3.4.4-antix.1-486-smp-10.00.Custom) ...