Partitioning simply makes life easier. Do you put all your clothes in one place or do you have a sock / underwear drawer, shirts drawer, pants drawer, etc. Some reasons to partition.....I'll go all out and use as many possibilities one might consider for most users
1. Speed - A HD is twice as fast at the outer edge as it is on the inner edge, so do you want things randomly placed all over the place or do you want the things you use most at the fast end and the things you rarely use at the other ? Your drive is 3/4 full now and you just got a new massive game..... you want that new game you just bought with umpteen GB of textures loading at the fastest possible speed or the lowest possible speed.
a. So putting the OS on a small partition (say 128 GB C:\ ) at the outside edge will have your OS files at the fastest part of the drive.
b. The next most critical thing from a speed standpoint is the machines use of temp and swap files. A small (D:\) partition = 32 GB will suffice .... If you use a swap file (as most do), it will sit here and never be fragmented as it would in a single partition drive. It's presence during program game installs also wreaks some minor havoc as it force game and program files to be spread out all over the place cause one day it's on one spot and another day it's in another.
c. Next it's decision time .... you want your games or programs loading faster ? I'll use games in this example. Set aside the amount you need for gaming on E:\
d. If games went on E:\, then programs go on F:\
e. Then we can store data at the back end on G:\
f. And finally backups (say image of C:\) on H:\
So now the things you want to go the fastest are on the fastest part of the drive and the things you don't care about are on the slowest part of the drive.
2. Backups - Instead of searching thru you HD looking for individual folders to back up, this is now very easy.
a. Have back up program run daily for Data partition, to set up click the one folder (partition)
b. Back up the other partitions (or not .... nothing there is irreplaceable) only when you add a new program or game.
c. Image the OS say monthly to the backup partition; if OS gets fudged, do a restore.
3. Upgrades - You want to upgrade, just format C and Reinstall / Upgrade OS. Since you didn't format the entire drive to do the install, everything else remains intact.
a. Install programs over themselves at original location to set up registry on new OS install; all program customizations, modified toolbars, etc will remain intact.
b. many games will still work, if not repeat as above.
4. Another minor speed tweak you can do is format the temp files partition to FAT32 instead of NTFS or GPT. The extra file protections of NTFS / GPT are valuable for files that matter but temp files don't. So not needing file protection, you can lose the overhead associated with NTFS / GPT and pick up a little speed along the way as to Temp / swap file usage.