If any one bulb fails the whole string would go out so the manufacturers use special bulbs which go short-circuit when the filament fails. This is OK if just one bulb fails (giving 240v/23 bulbs = 10.44v) but if a second one goes the problem worsens to 10.9v per bulb. Eventually this over-running causes further failures and the whole string goes dead-short across the mains :shock:
By fitting one so-called fuse bulb (a normal filament lamp), if this blows the whole string is safely killed - you know (in theory!) that this is the cause of the string failing so is easily found and fixed.