Kids these days. You keep both twrp and stock recovery img's on your internal memory. Then use flashify to switch between them if you want to flash it for whatever reason.
I've had an unlocked bootloader for around two years (S3, N5 and N7) and had either CWR or TWRP loaded. I have no need for the stock recovery in daily use. TBH I mostly used recovery for flashing ROMs and doing Nandroid back ups. And lately, loading all them SuperSu updates.
But ya, I've got ROM Toolbox Pro that can do flashing like that.