erase Flash memory before Programing

Question1.

when programming Application, there is no request for erase memory. why programming Bootloader requeset erase flash memory? how about Soft Device?

Question2.

I'm using nRF51. SDK11.

Can I program Application+BL+SD (merged to one hex with mergehex.exe) without remove pstorage page?

there is a memory area no t want to erase.