For me:

• It tells time and is always correct, also during daylight saving season
• It tells the weather forecast, I check it before leaving my house.
• I like that the alarm wakes me up with vibration mode, but not my partner (yes I wear my watch at night, but around my ankle, pro-tip)
• I like to easily read important notifications (I am quite strict in what it does show, so only texts from important people, my calendar and like bank notifications get through). Added benefit is that I don’t get distracted with other stuff because I don’t open my phone.
• I often lose my phone, my watch is able to find it.
• I use my watch to get simple navigation option when I go running/inline skating in an area I don’t know yet. Works better than the phone because I don’t have to hold the phone in my hand.
• I set it up so my partner can see where I’m at. When I see a notification from them asking where I am I can easily (and safily) start the tracking without having to pull out my phone.
• It tracks my heartrate, so I can do heartrate based training.

So is it really necessary? No obviously not, I will survive without it, but I do like it a lot and would miss it of it wasn’t there. But that’s how it is for me, that does not mean it will be the same for you.