Why do I need a Microsoft account?

Question

visual studio

I'm learning visual studio. I use Visual Studio ENG 2015 Enterprise - full version (with key).
In the Visual Studio menu there is an item File->Account settings. And then, respectively, Sing in / up.
Created an account on the microsoft website. I go to Visual Studio - Sing in. I enter my email and password.
And then I see the inscription - Perhaps "[email protected]" is used for more than one account. Which account do you want to use? And offers - Work account or Personal.
I can only log into my personal account. I choose personal. I enter the login password and see a window - enter the password for [email protected] I bring it in. The account is connected.
As a beginner - there are several questions, googling which did not give a clear and precise answer.
1) Microsoft writes that there is a work study and a personal record. What is their difference?
2) Why can't I log into my work account when I log in to Visual Studio?
Doesn't accept username and password. But I can enter personal.
3) In Visual Studio, I see 2 connected entries - Visual Studio profile and Microsoft account.
I want to manage my Visual Studio entry (manage Visual Studio profile). I get to the login.live.com page and then I see a form with fields - a country code and another field without a description. I choose Russia +7 and in the second field I write email [email protected] In response I get - Enter a valid email address, phone number, or Skype name. But I entered it. In addition, I'm already logged into Visual Studio using these credentials.
Well, if +7, then it is logical to enter my phone number in the second field. I enter - 9041111111.
In response I get - This Microsoft account does not exist. Enter another account or register a new one.
Those. Visual Studio accepted [email protected] and password, but with (manage Visual Studio profile) these data are not accepted.
What's the matter here?
These are the questions. Something strange with these accounts. I don't understand what they want from me.