Is sand really needed under an above ground pool?

Yes the weight of the water on the liner and feet on the pool bottom could rip or crack liner. The sand free of rocks is important to protect the liner.

Is sand realy needed under an above ground pool?
Tyvm

Yes , An above ground pool liner sits on the ground. The sand is under the liner above the ground.

How much sand would I need for a 15x48? And also is the sand expensive? TYVM

I have a 15x48 pool and ordered 2 tons of sand. I also need to level some spots off. If you don't need to level, then 1 ton should be more than enough. The 2 tons cost me 115 dollars.

There are other options but you do need to use something. The earth is just too difficult to sculpt and level so sand is a cheap option. You need to protect the liner from the normal debris in the soil so the liner will not get pierced.