Why do long leasehold flat leases need to be extended?
Historically, flats were mainly sold off on 99 or 125 year leases. Leases get shorter with each year that passes. Even though the value of the freehold residual interest will typically be low because there is a statutory right for long leaseholders to keep extending their leases (so the property never reverts back to the freeholder), it can be a hassle and expense to extend your lease.
One of the main reasons why leasehold flat leases more commonly now need to be extended is because the policies of mortgage lenders have changed so that whilst previously a lease length of twice the mortgage length (so 50-60 years usually) was acceptable, now most mortgage lenders will only lend where the remaining time on a lease is at least 85-90 years.
The most common reasons for extending your long lease are in preparation for selling your flat, or if you want to (re) mortgage your flat.