If the property rented out for £10k per year and you could buy it for £15k. This would be the benefit; or if you were happy to rent it for that period of time and the price suited you. The other benefit is that your rental price is locked for the entire period wheras shorter tenures are likely to see price rises at least in line with inflation. Enfranchisement would be massively expensive. The freeholder gets possession in 20 years so the price of the extension will not be cheap. When a lease has 80 years or less the (marriage) value rises with each passing year. If you are loaded and you are expert in the valuation of lease extensions. There may be profit in it.
With leasehold property's if there is not many years left on the lease then they are difficult to sell and very expensive to extend. Also with leasehold property's there are a lot more restrictions on what you can and cannot do. Frankly speaking there are not many advantages of a leasehold property.