Because no matter how prepared I think I am, there's always at least one item on the ballot I didn't expect to see.
I can sit down at my computer and look up everything on my ballot and make better informed decisions in the comfort of my home. I can lean over and ask my spouses opinion on what they're voting for and we can decide what we want to do. In the end we each make our own decision, but at least we can discuss it.
I have a QR code on my envelope that shows that I can track my ballot from end to end and ensure that it's where it is supposed to be. If it never arrives, I can go vote in person and have a provisional ballot to ensure I didn't vote twice.