Both, some hope to gather bank and other info if they gain access to your site and may try to blackmail you after they take over for you to regain control over your site. They may still leave a present on your server which may still allow them access later. This is why regular backups are truly needed in these times. You can always go back to a safe backup, IF you know when they originally took over and placed rogue software on your server.
We do daily and weekly backups on the hosting just in case we screw something up or something like this happens. However, it is really each user's responsibility to keep their own backups. What if ours fail?