Hi, I go to explain to you in two environments:
1 enviroment: You have a 1TB HDD WITHOUT partitions, in this HDD you have documents, movies, music, games...all. Some day the disk fail, you can't recover the OS and have to install the OS again.
Final result, you lost all your documents, movies, music, games...'cause you need format to install the OS again.
2 environment: You have the same 1TB HDD BUT WITH 4 PARTITIONS:
C Partition: OS, Documents and programs
E Partition: Games
F Partition: Music, videos and others
G Partition: Documents backup
Some day, you start the PC and the E partition fail, so, you need format the partition and create again.
Final result, You only lost the games, your documents, videos and music are safe in the other partitions.
Finally, you can select the partition in wich you want add the page file, this can increase the performance.