Does every PIC need a bootloader?
As others have stated, you don't need a bootloader for PIC microcontrollers (and most others from other companies, in fact I know of no microcontrollers that require one).

One reason to have one however is to allow firmware to be upgraded without needing to connect up a programmer like the ICD3 or a REALICE.  (Some products are designed with headers that allow a programmer to be connected after the product is put into a case, but most are not.)

I have used two different ways of upgrading firmware without a hardware programmer.  Both require bootloaders to be flashed into program memory, usually at the very beginning or very end of program memory area.  Ideally, once the bootloader is flashed, then this portion of memory can be write-protected, so an errant application program cannot erase the bootloader.

One way to upgrade firmware is to provide the application on an SD card, and have a slot in the product that allows this to be accessed.  Each time the microcontroller starts up, it checks to see if the application on the SD card is different from the one in flash, and if so update it.  (You don't want to blindly download and flash the program each time since flash has a limited number of erase cycles.)   This requires the bootloader to have the code necessary to access the SD card (which will probably have a FAT16 or FAT 32 file system on it.)

Another way to upgrade firmware is FOTA -- firmware over the air.  In this case, the firmware is downloaded over some wireless connection, such as a cell modem, bluetooth, or Wi-Fi.  It is then flashed into program memory.

A variation is to combine the two, whereas the new program is downloaded and written to an SD card, since it may not be practical to flash the program as it is being downloaded.  Note in this case the bootloader doesn't need to have any of the code to access the wireless network, as the application program can download the code and write it to the SD card before it switches to the bootloader.

The PIC32 is particularly suited to having a bootloader, as it has a 12 KB segment of memory specifically set aside for this purpose.