Why Foam dielectric is mostly air bubbles, so there is less solid plastic to absorb RF energy as heat. That lower dielectric loss means foam cable typically shows noticeably less attenuation per 100 feet than solid polyethylene cable of the same size, especially at VHF and UHF. Foam also has a higher velocity factor, around 0.78 to 0.85 versus about 0.66 for solid PE, which matters when cutting phasing or matching sections.
Watch out Moisture resistance and voltage breakdown are actually the weak points of foam cable, not advantages: the porous foam wicks water more readily and the thinner effective dielectric withstands less voltage. Impedance is set by the conductor dimensions and dielectric constant together, so both types are made in 50 ohm versions.
Foam = more air, less loss. Its trade-offs are moisture and voltage handling, so any answer praising those is wrong.