Why do we need a preloader and a bootloader in many applications before even the main program
Diagram of Loading Process



It doesn't sound like you read very far into the datasheet.  From page A-4:

Preloader

The function of the preloader is user-defined. However, typical functions include 
initializing the SDRAM interface and configuring the HPS I/O pins. Initializing the 
SDRAM allows the preloader to load the next stage of the boot software (that might 
not fit in the 60 kilobytes (KB) available in the on-chip RAM). A typical next software 
stage is the open source boot loader, U-boot. The preloader is allowed to load the next stage boot software from any device 
available to the HPS. Typical sources include the same flash device that contains the 
preloader, a different flash device, or a communication interface such as an EMAC.

This is from the datasheet pcj50 linked.