What the restriction is doing there
It is how a landlord or management company makes sure it finds out who owns each flat and that each new owner is bound by the same obligations as the last. Without a restriction, the obligations in a transfer would be easy to leave behind on a sale.
The Land Registry enforces it mechanically: the transfer is not registered until the certificate is produced. That makes it one of the few requirements in conveyancing nobody can simply decide to ignore.
The fees, which are small and numerous
A fee is payable for the certificate, typically to the managing agent, and often per certificate — so a purchase with a mortgage generates more than one, because notice of the charge is given as well as notice of the transfer.
These sit alongside the other management company fees that arrive in ones and twos: notice of transfer, notice of charge, the management pack. Individually small, collectively worth knowing about before completion day.
They recur when you sell, in the same way.
When it goes wrong
The problem case is a landlord or management company that does not respond, and it is the direct consequence of the problems on the other pages in this topic: a dissolved company or an absent landlord cannot issue a certificate, and the purchase cannot be registered.
An unregistered purchase is a serious thing rather than a technicality. The legal estate has not passed, the lender's charge is not protected, and putting it right later is substantially harder than doing it at the time.
If the certificate is going to be a problem, that needs to be known before exchange, not discovered afterwards. Ask whether the landlord or agent is responsive, and whether anyone has had difficulty with a recent sale in the building.