Sending Notifications

1

I am new to Home Assistant, and to YAML. Could someone please tell me
the difference between the following two snippets of code? I created
one with a device action, and the other using mobile_app_pauls_iphone.
They appear to do the same thing and I’m wondering when I would use one
or the other.

Thanks in advance.

alias: Send Message to Paul's Phone
sequence:
  - device_id: 7ce1d16b010acfe0c94cfd8ef7a89e72
    domain: mobile_app
    type: notify
    message: Test Message
    title: No Title
mode: single

alias: Test mobile_app_pauls_iphone
sequence:
  - service: notify.mobile_app_pauls_iphone
    metadata: {}
    data:
      message: Test Using mobile_app_pauls_iphone
  - device_id: 7ce1d16b010acfe0c94cfd8ef7a89e72
    domain: mobile_app
    type: notify
    message: Test using device
mode: single