Why documents are asked for
Phone numbers are not an open resource that anyone can mint. A regulator allocates blocks of numbers to a licensed carrier; the carrier makes ranges available to a provider; the provider offers individual numbers to a business. Any verification requirement travels down that chain — regulator, to licensed carrier, to provider, to buyer — so a requirement you meet is one set above the provider, not invented by it.
The purpose is anti-fraud and consumer protection. Knowing who holds a number makes it harder to use ranges for scams or spoofing, which is why some markets ask a buyer to prove identity before a number goes live. This is what KYC (Know Your Customer) means: confirming the customer behind the number.
A key point: the requirement attaches to the number range, not to the provider. Two ranges in the same country can carry different rules, and the same range carries the same rule at any provider. Because of that, the exact requirement for a number is known in advance and is shown to you before you order, so there is never a surprise after purchase.