(2-part post):
When someone purchases the leasehold on a property, they're not buying perpetual ownership of the building. All that their money is getting them is fixed term ownership of the property. Thereafter, ownership of the property reverts to the freeholder
For example, if someone takes out a lease on a shop, they might pay �50,000 for a 5 year lease, with payments of �500 per month (as 'ground rent') throughout the five year period. At the end of that time, the freeholder can again offer the lease for sale, at whatever figure he chooses. (That's why you often see signs stating 'Closing down - End of lease'. The trader has decided that the price to renew the lease is too high for him).
If the trader, in that example, wants to move out a year early, he can sell the lease but he won't get �50,000 for it, because it will only provide the purchaser with 'ownership' of the shop for a single year. So, he'll be looking for around �10,000.