Registering for Online Account

I've been doing paper returns for around 20 years. I have a day job where I pay PAYE and I'm a landlord, for which I do a self assessment. I'm not too clued up on online matters but think it's time to go online. I've already got a UTR but when I watched online videos, they say - customers will be sent a UTR by post. I can't understand this. If I've already got one, why would I need a new one? Hope someone can clarify.

Hi amka,
As you already have a UTR and a live Self Assessment record you do not need to register for Self Assessment instead you would enrol to file your tax return online.
To do this go to:
HMRC online services: sign in or set up an account
Click 'add SA' and then use your UTR number with your NINO or post code to enrol.
This will trigger the issue of an activation code.
You will receive the activation code within 10 days (21 days if abroad).
Use your activation code here:
HMRC online services: sign in or set up an account
You can then access SA via your PTA landing page.
Thank you.