Because Premium is a one-time purchase of a defined access period rather than a permanent entitlement, and because practice material for a networking certification exam is only worth sitting while it still matches the objectives you are being examined against.
It is worth saying what the period is not. It is not a countdown engineered to hurry a decision, and there is no penalty of any kind attached to reaching the end of it. It is simply the shape a single payment takes when the objectives underneath it do not stand still.
A Fixed Period Is What a One-Time Premium Purchase Looks Like
The usual alternative to a defined period is a subscription, and NWExam does not run one. You pay once, the period runs, and at the end of it nothing renews, nothing is charged and there is nothing you have to remember to cancel. That is a deliberate trade. The cost of it is that access stops; the benefit is that it stops by itself rather than continuing to bill you because you forgot it was there. If you want more time later, buying again is another one-time purchase on the same terms.
Why Blueprint Currency Matters More Than Length
The second reason is the one that justifies the first. Networking blueprints move. Segment routing sits on service-provider objective lists that did not name it a few revisions ago, and an objective list can gain and lose whole sections between two revisions of the same certification code. Practice written against a superseded list rehearses the wrong material, and rehearses it confidently. That is an argument for sitting your practice inside the window in which you are actually preparing, which is what a defined access period gives you.