Confused about setting up segment registers

I am working on writing a bootloader, and the following tutorial introduces this code:

main:

 ;----------------------------------------------------
 ; code located at 0000:7C00, adjust segment registers
 ;----------------------------------------------------

      cli                       ; disable interrupts
      mov     ax, 0x07C0                ; setup registers to point to our segment
      mov     ds, ax
      mov     es, ax
      mov     fs, ax
      mov     gs, ax

 ;----------------------------------------------------
 ; create stack
 ;----------------------------------------------------

      mov     ax, 0x0000                ; set the stack
      mov     ss, ax
      mov     sp, 0xFFFF
      sti                       ; restore interrupts

Maybe I misunderstood something, but if the SS register contains 0x0000, this does not mean that the the ds, es, fs and gs will overlap the stack? Also what are the functions of the fs and gs registers? Also, does the cs segment automatically install the BIOS? Because he says the code is at 0000: 7c00. In addition, the tutorial does not say why interrupts are disabled. I read somewhere that interrupts are usually disabled to avoid a dead end. What does this mean and why is this happening?