Its very simple to understand that if you are using the partition to store th data where the OS is installed then if any problem will occure and if need to format the partation then your all stored data on that perticular drive may loss. so to avoid this it is recommended to have new partition to install OS
beyond that day by day the size of partition goes increasing where OS is instlled and it require more space to install more applications and hense data should stored in different partition....
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