Installing the cylinder of a euro lock may be necessary if a set of keys has been lost or you move into a new property and want to be confident that there are no keys unaccounted for. Or you may want to change the style of lock to one with an internal 'thumb turn' knob - this is ideal where you do not want to be locked out of a room (such as a child's or old person's room. Alternatively you may want to change from a 'thumb turn' lock to a double key style - probably advisable for a front door where there are children in the household.