So after discussing with the querant in the comments, it seems that they were confused by the terminology
In computer storage parlance,
partition refers to a data structure placed on the hard disk which contains a
filesystem. Partitions can be of any size, from a very tiny size to occupying the entire disk volume. But even if you only want one partition with one operating system on your hard disk, you still need a partition.
So don't think of partitions the way a truck driver thinks of them with a multi-fuel oil tanker. The truck driver thinks they have three partitions (Regular, Plus and Premium grade gasoline) divided by two walls in the middle of the tank. But if the truck driver was only carrying one grade of gasoline, there'd be no need for partitions and they'd have an oil tanker without those dividing walls in the middle. That is not how hard disks (or any computer storage media) works.
The Master Boot Record of the hard disk describes the partitions on the disk. Alternately, newer computers use the
GUID Partition Table instead of the
PC-MBR, which is a legacy IBM format from the 1980s. The GPT, instead, is much more flexible and supports newer computing technologies and larger capacities, as well as more partitions on a single disk. But the old
PC-MBR and the
GPT are both considered to be different formats (or types) of Master Boot Record (MBR).
So, every useful hard disk has to have some type of MBR, and the MBR defines which partitions are on the disk. Even if you only want one partition with one operating system, you've got to have these things, even if the partition says "I want the whole disk". You can think of a single-partition disk as being the walls of the oil tanker's container itself, spanning the entire volume that the container holds.
The important point to remember is that, in computing, a partition does not imply that you must have two or more separate spaces on the hard disk.
Also, as some added insight, be aware that all modern operating systems either require or highly recommend that you use at least two separate partitions. These partitions are often recommended or created automatically by the operating system installer, and the partitions serve different purposes.
On Windows, it often creates an
EFI System Partition (on new UEFI PCs or Apple Macs) from which to boot, as well as a
System Reserved Partition which is not normally accessible to users but is used for system infrastructure. It then creates the
System partition (not to be confused with the
System Reserved Partition which is where your C:\ drive is, and thus contains all your data as well as the operating system itself.
On Linux, it often creates a moderately-sized
root filesystem which contains your core operating system files; a very small
boot partition which contains the Linux kernel and the boot loader (e.g.
GRUB); a large
home partition which contains the /home directory and usually occupies the majority of your disk space; and a small
swap space which is like Windows' page file a.k.a. virtual memory (a complicated topic but let's just say that it is highly recommended to have one but not required).
Partition schemes are, for the most part, arbitrary, and the primary purposes of creating partitions at all are as follows:
Allowing different filesystems to be used for different purposes. For instance, on Linux, you might want the
reiserfs filesystem to be used for your smaller
root partition due to its fast read performance and efficient use of space on small files, but you might want the
xfs filesystem on your large
home partition due to its higher scalability with large amounts of data.
Creating a security model for files not intended to be seen/accessed by the user to be hidden away. This is mainly the purpose of Windows'
System Reserved Partition.
Separating out data of different types; for example, the operating system's executable files and data files comprising your programs can reside on the
root filesystem, while your user's data (documents, music, video, web browser settings, etc) resides on the
home filesystem -- separation of concerns. That way, in case you want to reinstall your
root filesystem with a new distribution of Linux, you can retain all of your user's personal data without having to back it up to a separate storage device first.
Running multiple operating systems on the same hard disk -- This is fairly obvious, but partitions are almost always required when you want to run two or more separate operating systems from the same physical hard disk.