You have four primary partitions and want to add a fifth... and you can't just redeclare them extended/logical because those need an extra sector for each partition.
Also GPT has a backup at the end of the disk so if you ever lost a partition table to MSDOS and had to resort to TestDisk, with GPT you might be able to do without.
grub on BIOS system with GPT partitioned needs an extra 1MB partition
It's closer to 64KB actually, at least it was around that when I last checked what was actually written to that partition. And with msdos partitions as well, grub has to put its core somewhere. The only difference is that with GPT the grub developers thought it'd be nice to make it official-like by having a dedicated partition type for it.