So you got your ballot in the mail, read the instructions, managed to find and read the law on LIS, and already know you need a witness to sign it when you posted this.

When you vote in person, multiple election workers are there to confirm you're there by taking your ID, having you repeat what's on your ID to confirm your voter registration and your ID match*, they then repeat that back to you when checking you into the system and handing back your ID. Then, you get a receipt to hand to another election worker who hands you a ballot and keeps the receipt to be scanned, and then that ballot is scanned in and stored until Election Day when the vote is recorded. Everything is linked up for that vote against your voter registration with a chain of custody.

They can't do that when you do a mail-in ballot, so the witness is standing in for the election workers. Basically, you and your witness (who can literally be anyone) are attesting to the fact that someone else did not vote for you through an honor-based system of illegible signatures that generally mean nothing but can absolutely be challenged by poll watchers (not election workers).

Your most reliable bet is to vote in person and leave the witnessing to the election workers.

So, what's the question again?

*If none of this matches or they can't find your registration for any reason, you're provided a provisional ballot that requires additional reviews and possibly additional verifications you absolutely must do during the "curing" process. A curing process also exists for mail-in ballots if they're challenged by a poll watcher and extra reviews are needed. If you don't follow those extra steps in time, your vote isn't counted.

Edit: grammar and context