I do; there’s even an option in settings to mark a certain contact card as your own, so it’s a behavior that Apple seems to anticipate.

Yes, you are supposed to do that. Then you go to Settings > Contacts and select your contact card for My Info.

This is how Reminders knows when you tell it to remind you of something when you get home. This is how Maps has the Home and Work buttons with the correct address. This is how people can find out who owns the phone they just found by asking Siri “whose phone is this?”

I do - I use it to look myself up quickly text myself things to read or look at later in leu of saving elsewhere… e.g. links to articles and such I want to read later

As much as it might be confusing, this is the way to go. You can add your birthday, mark your family members etc. Siri learns from this and provide useful prompts (navigate home/to work, call my mother, etc).

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That's where the device gets the information to fill out all the forms you have to fill out.

Yes and yes. And then you select it as your contact in settings- contacts - my info. Like this the phone can understand where to take you if you say ‘ take me home‘ and stuff like this (remind me to wash my hands when I get home, how far am I from work, call my wife …)

Yes, it’s what populates “auto fill” on websites etc.

I do - and in addition I have a few additional ones, in case I need to share but don’t want to reveal (share) all information (as an example; have a number of different e-mail addresses and numbers, and not everyone should know all of them).

If my contacts got their info stolen from me, they are already dead.

It’s incase you forget your number and can’t get ahold of yourself