For me this is my use of the notification light.
Usually I am working around a desktop (editing videos). I will hear my phone notification sound go off indicating I have an email, whatsapp message, etc. I would say "hmm okay I will reply as soon as I finish editing this video". Majority of the time I will then forget I had a notification when I am using my iPhone as a daily driver (perhaps because of my bad memory lol). However when using an android or blackberry I will have that constant light blinking, so in essence a constant reminder.
Now obviously you may be different from me, but this is how a notification light becomes a big deal for me. So instead of having to check my phone every 15 minutes, I can just glance at the phone without moving my hands from my workstation.
Not all of us wants to use a smartwatch. I have never used one and don't intend to. I don't want a smartwatch to be a solution when I could have a measly little led light as my solution. The led light does not need charging, or do I need to remember to put it on, or to pay extra for it, etc.
Again this is MY use case. Don't try to force down your methods of "you can just unlock your phone and check notifications". This is by far less convenient for me.