Portable bathtubs are useful for a variety of reasons. Sometimes they are necessary for someone who is bedridden, they can be a space-saving option for small homes or they can be used temporarily during an bathroom remodel. They're also extremely useful for camping trips or outdoor bathing in general, which is how most portable tubs are marketed.
A subset of portable tubs is directed at people who are invalid. These tubs are very shallow, and can be laid on a bed and then inflated around them. Because they are medical equipment, they tend to be expensive and are mainly intended for use by people who treat invalids, such as hospice workers.
Inflatable Camping Bathtubs
Deeper inflatable tubs are available. While marketed for people who camp, these also can be used indoors. The trouble with deep tubs made of plastic, however, is that they are not particularly durable, and a dog, cat or anything sharp could cause a flood in the house. Inflatable tubs have their limitations, but used correctly they can be effective.
Waterproof Cloth Tubs
Another category of camping bathtubs are cloth bathtubs set in some kind of frame. Sometimes they have covers that can be zipped up to provide a fully covered bath, in the case of bathing in a more public place. These face the same limitations as inflatable tubs in that a sharp object could cause a mess if the cloth tubs were used indoors.
Hard Portable Tubs
Freestanding tubs made of wood or acrylic material probably are the best option for in-home use. They can be stored in another room when not in use. They don't offer the space-saving capabilities of inflatable tubs, but they're much more durable -- and safer when it comes to pets and kids.