That was faster than I expected. The leaked ISO’s have been cracked, allowing users that downloaded and installed the illegal copy to activate Windows. The new crack bypasses the KMS servers, which allow a 180 days before it checks against a valid server again (which the crack can again be applied).
Although the leaked ISO is out there, I do not recommend getting it for an advanced copy. Sure, it may be a legit copy. But, there may also be some infected copies out there. Nothing like a fresh OS install with built in malware and root kits. That is completely aside from the legal and moral aspects.
Activation will allow you to customize your Start Screen and other various things that require activation before allowing you access to those features.