Why does PPSSPP emulate the PSP operating system?

[Squall]

I think the emulator should emulate the hardware and execute the software.

Operating system is a software and it should be extracted from an original PSP and PPSSPP should emulate the hardware and execute the extracted OS.
If that Atract3 codec is included in the operating system, then it would not be needed to emulate it.

I have this question because some emulators, such as PS1 and PS2, require BIOS from the console.

[[Unknown]]

ATRAC3+ is included in the hardware, so that won't help.

HLE emulation can be easier than LLE emulation, and gives us a lot more room to make things efficient to get better performance. Also, I don't think it's practical to expect everyone to rip the firmware from their PSP (which is harder to do than ripping games.) I realize that some people pirate it, but when considering how practical something is I only consider those that follow laws.

-[Unknown]

[VIRGIN KLM]

The only usefull thing out of the whole PSP Bios are the fonts (and possibly a couple of modules) that PPSSPP have for both cases substitutes for them that makes zero change at all.
LLE Emulation for the whole hardware is like an insane project, which would render the name "PPSSPP" pointless since a high end PC would barely be able to emulate games at anything over 1-2fps, let alone Android phones.
Partial LLE emulation though for the ATRAC3+ decoder though could be possible but it's not going to happen since nobody knows anything about the hardware that does this job plus ATRAC3+ reverse engineering is an easier task to do than doing this.

[Henrik]

I think LLE emulation could be reasonably fast on a PC at least, but the main problem is that unlike on many other platforms, very little is known about large parts of the PSP hardware, and not all parts of the kernel have even been decrypted yet (although there may be ways around that).

HLE is great for speed, compatibility is indeed a bit of a pain though, but it can be when doing pure HW emulation as well.