I was responding to the posts stating they had not needed to provide an NI Number before and your, "This sounds a bit fishy to me." comment.
I gave the reasons for needing the NI Number in the specific circumstances quoted in the OP and I was not generalizing.
Regarding the use of an NI Number to access HMRC account and obtain personal information, the NI Number alone would not be sufficient to set up or access an HMRC account.
Regaring the information in the personal tax account, the guidance says it can be used to:
•check your Income Tax estimate and tax code
•fill in, send and view a personal tax return
•claim a tax refund
•check and manage your tax credits
•check your State Pension
•track tax forms that you’ve submitted online
•check or update your Marriage Allowance
•tell HMRC about a change of address
•check or update benefits you get from work, for example company car details and medical insurance
•find your National Insurance number
Other than State Pension, no mention of any other State benefits.