What are the 2 new dummy outputs created for?

[jadwahab]

Firstly, thanks for putting this together!

I'm just a little confused about what the 2 new dummy outputs created are for. I see the code says: https://github.com/magiceden-oss/msigner/blob/3de81d4aaac4d030b2fb2967d704a59070981777/src/signer.ts#L465

Is it to avoid having to create a new tx with 2 dummy inputs in a future purchase? Also, do they have any specific values?