Suppose you have a customer that needs a /22 and they want to go
multi-homed. Suppose you give them that /22 out of your /19 or /16 you
got from the RIPE NCC. So they announce their /22 to you and to their
other provider. But you keep announcing your /19 or /16. So if anybody
were to filter the /22 announcement, your customer only suffers partial
loss of connectivity, since you are still announcing an aggregate of
their announcemnt (your original /19 or /16).
Problem fixed. Anything else?