If you’re building a room in a basement, you’ll have some walls that are built next to the foundation. We’re not worried about sound leaking through the foundation into the surrounding soil. We’re worried about sound traveling straight up into the floor joists.
If you were to build no wall against the foundation, your ceiling will have to be lowered sufficiently to have the ceiling drywall directly intersect the foundation, and then sealed to the foundation. That’s fine, but you lose a few inches of ceiling height. Most would prefer to not have a wall or two be cement, so they plan to build a wall anyway.
Not addressing this area behind the wall (exposing sound to the joists) is a classic flanking pathway. To better understand flanking path review the article Understanding Flanking.
For soundproofing theaters and recording studios, all walls, ceiling, and floor (unless it’s a slab) need to be treated or you’re largely wasting time and money. For soundproofing condo walls or ceiling, the sound levels are lower (lower dB) and typically of a higher frequency, so a single wall or ceiling-only solution can work fine and reduce a great deal of noise.