Common examples are someone is sent a contract of employment unfortunately often after starting and they don't sign it. If they have been coming into work broadly in line with that contract so long as it's fair, employee and employer are bound by it.
Here is an interesting edge case in the UK [0]. Long story short if you give someone the ability to sign on your behalf and appear to consider parties bound by that it's binding.
In a personal sense, I send out appointment letters but my secretary does it for me. I consider myself as bound to that as if I arranged it myself. I gave the secretary latitude to book appointments for me, and I usually turn up to those appointments. If that letter is signed, or if it's by my hand. Doesn't legally matter as much as you'd imagine.
[0] https://www.lexisnexis.co.uk/blog/banking-and-finance/gordon...