I haven't had to sign for a while now, but when that was more common, I just assumed it was for your own ease of verification, since you could see the signature

I haven't had to sign for a while now, but when that was more common, I just assumed it was for your own ease of verification, since you could see the signature alongside the transaction online/in-app and so if you see a transaction you don't remember, you can just check the signature to see if it's like what you typically do.