There are various different types of bath rails available to aid someone getting in and out of the bath, either alone or with the assistance of another person.
Some can be easily clamped into place and can be adjusted, they can come in metal or plastic and other designs can be fitted to the wall or closer to the taps, it depends on where the user will need the support of the rail the most and so some of the rails are designed to be specifically placed in a particular area on or close to the bath.
They can also come with suction cups attached to make repositioning of the rail quite easy.
Even for an able bodied individual it can be a little dangerous at times when stepping into or stepping out of a bath and its quite easy to slip and fall, so in general a bath rail can give a safer and secure feeling to just about anyone when getting in and out of a bath.
Where a person does have a disability and does require the need of a rail, it can provide support for an individual who is quite independant and prefers to do things for themselves and may prefer to not have the assistance of another person while using a bath but at the same time they have that confidence that the bath rail is there when they most need it.