Why create many partitions?

"There are several reasons:

System robustness. If you have /home on a separate partition from /, then a regualr user can't fill up the / filesystyem, making it unusable for the rest of the system.
Backups. It makes it easier to back up entire partitions, and to back up on different schedules. For instance, you might only need to do weekly backups of the system partition, but nightly backups of the /home filesystem
System installs. You can have the same /home filesystem mounted by several different system images. And you can delete and rebuild / with out doing a backup/restore of /home or /local
Disk optimization. Not as much of an issue with todays fast drives, but it used to be common practice to put the system filesystem on the inside tracks of the disk to speed up access
Using multiple drives. Before the common availability of large drives, it was common to have little room on the system drive. So a separate drive was used for /home.
NFS. When sharing data across multiple systems, it is common to do it on a filesystem basis."

"Keeping a separate home partition lets you keep your files should you reinstall your linux distro.

GRUB doesn't support ext4, so if you're going to use that filesystem for the rest of your files, you'll need a separate boot partition. GRUB now supports ext4, so keeping a separate boot partition, while still an option, is no longer necessary."

Linux generally sees the disk very differently than a Windows machine. Also Windows Fat32/NTFS and even newer file systems are mostly a Windows strictly format. So you don't actually know what they do or how they behave internally. Linux uses its own file systems ext3/4 being the most widely used, the Linux file system doesn't exist in a tree like state nor does it treat data like a second class citizen residing on your disk. The reason you can create single or multiple partitions for the os is both for organization and disk speed access. Though this last part has changed over the years, most Linux users don't bother with multiple partitions because you need to slice your os data to fit evenly over the partitions you manually create. I personally find it counter intuitive to have multiple partitions. On the other hand Linux machines don't generally suffer from bad disk sectors or having an auto feature for defrag. This is just bad design by Microsoft, can't fix their internal issues so they build tools that allow you to fix your own problem 60% of the time anyway.