Value of a leasehold property depends to a considerable extent on the length of the lease. If a lease was never extended then when it ran out the leaseholder would have nothing - the whole value of the property would revert to the freeholder. The nearer that event is in time the less anyone will pay for the leasehold interest (& anyone buying a property with only a short lease will not be able to get a mortgage).
So when a lease is extended it increases the value of the leasehold interest in the property. The landlord (i.e. the freeholder) gets a share of that increased value & paying that share is part of the cost to the leaseholder of extending the lease.