To be more attractive (to sell more)?

My understanding is that grafting was done to grow cacti which are very rot prone, slow growing, or generally difficult to cultivate on their own roots. Some plants when they get large enough and with some care CAN be degrafted, some cant.

Commercially its to create variation, its very mad scientist to me.

They pick root stocks that can take more abuse and over watering. Cactus grows roots very slowly and rots easily if over watered. By grafting, the cactus tops grows larger and faster and can be removed and rooted on their own in time. Except for the color no chlorophyll types. At least that's what I was told.

Good point! That is why orange trees are grafts onto wild orange root stock. The wild tree can grown as it is resistant to the pests in Florida's soil and the graft produces the sweet fruit we enjoy.

Though I think the marketing that Christopher mentioned is also in play.

I've found Hylocereus to be particularly difficult to get the right balance of water, very, very easily rots...care's much easier with the roots of the plants pictured (that I've had experience with, at least).

Comparison charts from this site - according to them, Hylocereus rarely rots and has high water tolerance:

-hylocereus and other grafting stocks-

Hylocereus undatus

Hylocereus undatus

Full sun

10c +

Tolerates over watering

Easy-Medium grafting difficulty

Very fast growing, permanent stock

Advantages

1. Fairly easy to graft with.

2. High water tolerance; rarely rots.

3. Very fast scion growth.

4. Easy to propagate.

5. Cheap.

Disadvantages

1. Bends under the weight of heavy scions.

2. Does not take cool temps.

2. May split hard skinned sp. (Astrophytum).

3. Some scions become "bloated" (ugly)

4. Some species are hard to graft onto this sp.

Notes: Hylocereus undatus is a very popular stock used by many commercial and hobby cacti growers. It has huge advantages as it rarely ever rots. It is from tropical climates that often flood so over watering it is virtually impossible. It can take any kind of light, but does well in almost full sun. It is fairly easy to graft harder skinned scions to this specie (such as Astrophytum) but softer species such as Lophophora do not take as easily and may waste away to nothing.

Keep grafts at 15-35c.

Keep grafts tape on 5-10 days.

We prefer this for the following species: Ariocarpus, Astrophtyum, Aztekium, Copiapoa, Corypantha, Frailea, Echinopsis, Gymnocalycium, Mammillaria, Obregonia, Ortegocactus, Strombocactus, Thelocactus, Turbinicarpus.

Rina

Kris, I agree the plants in the picture above can all be grown on their own roots hence the mad scientist label, but plants which are more rot prone like Aztekium ritteri/hintonii and blossfeldia liliputana for ex. those arent gimmicky imo.

I agree, Minty. Some cacti really benefit from the graft. But for some in the first photo, it is totally unnecessary.

Christopher