You can use it with or with out. I use mine with a saddle cloth to give him a bit more comfort on his back. I probably dont need it but he feels more comfortable in it.
I use a thin cotton pad between the half pad and him! Keeps the pad clean, but still gets the padding from the half pad. His saddle was fitted slightly wide with the intention of using a pad underneath it. A correctly fitting saddle can be made too tight with the use of one.
You see people using them with or without numnahs.
I'd ask why your friend thinks you should use one, if it's because the saddle doesn't fit then you would be best to spend your money on getting the saddler out!
They are in fashion at the moment, but if your saddle fits correctly then by putting a big thick pad underneath it, all you are doing is making it too tight.
It's like wearing shoes that fit you perfectly but then sticking inner soles in them just because everyone else is
It's not a huge padded thing, just like a normal cloth, but obviously smaller.
I don't use anything else with it, and the reason I use it is because I had problems finding a pad, cloth, numnah that didn't pull down under her saddle.
The half pad is great and doesn't budge an inch.
Only down side, is you do have to wipe the underside of the saddle flaps more.