Today, it doesn't really matter if you use primary or extended partitions.
There have been times where /boot had to be a primary partition but this isn't true any more. Also earlier incarnations of MS Windows required that you installed Windows to a primary partition.
When I set up a new disk, I make the first partition primary and put /boot there and put the rest as logical partitions into one big extended but this is just a personal preference.
Partitioners like to align partitions on a mebibyte boundary these days. For MBR partitioning, there are 4 primary partitions, and for the rest you need extended and logical partitions.
While the layout of the primary partitions is expressed at the end of the first sector of the disk, for the logical partitions, you've got a linked list of additional partition tables (themselves specifying only one partitition. Typically, the first one is as the beginning of the extended partition (which is itself defined as a primary partition) and defines the first logical partition, and it links to the next partition table which defines the next logical partition. That next partition table will be located typically after the first logical partition.
All those partition tables only take a few bytes outside of the partitions, but because of the mebibyte alignment, a full mebibyte has to be used for them.
GPT on the other end stores all the partitioning information at the beginning of the disk (with a backup at the end), so after converting, that space that was used for the logical partition partition tables becomes free.
Note that you only need one sector to store those MBR logical partition tables, so strictly speaking, in MBR partitioning there would be 2047 sectors free, if the partionner was willing not to align partitions on mebibyte boundaries.
Best Answer
Historically, hard drives were only been able to contain at most four partitions because of the originally defined format of the partition table. This is not specific to operating systems. You simply can't create more than four primary partitions under the original PC partition scheme (MBR partitions).
In order to circumvent this limit and still remain compatible with older systems, you can create an extended partition however. An extended partition can contain multiple logical partitions within it. This allows you to create more than four partitions in total, without having to change the format of the partition table.
If you're interested in the details, you can look at the Wikipedia entries on disk partitioning or the master boot record.
This limitation and the concept of extended and logical partitions do not apply to other partitioning schemes such as GPT used by UEFI on recent PC.