Stairlift is a relief for those who can not clamber up and down the stairs due to a reason. These people include the old or the afflicted persons, who without any support can not climb up or down the stairs.
Straight stairways are common configuration for stairs that do not have any bend, curve or landing at some point. However, in most houses, stairs turn either left or right at the bottom or at the top. Similarly, those who have spiral staircases or staircases with middles landing, all need curved stairlifts for their staircases.