Bertha Jorkin's Memory Charm

[nera at rconnect.com]

No: HPFGUIDX 18123

Voldemort says concerning Bertha Jorkins:
"We could have modified her memory? But Memory Charms can be
broken by a powerful wizard, as I proved when I questioned her. It
would be an insult to her memory not to use the information I
extracted from her, Wormtail." 

I do not understand this paragraph. What did Voldemort mean when he 
said, "as I proved when I questioned her." ?

Who had put a Memory Charm on Bertha Jorkins? Why would they have put 
a Memory Charm on her? 

This is one of those things that I wondered about at the time I read 
it ... then I got distracted and forgot about it.

Can anyone clear this up for me?

Doreen