Generally they’re for uneven surfaces like multiple chips on a PCB (RAM, SSDs, etc.) or where there’s a gap between the chip and the heatsink (e.g. something using the chassis as a heatsink).
There’s also thermal epoxy, which glues the heatsink to the chip, like with smaller cards that don’t expect to have the heatsink removed.
Generally they’re for uneven surfaces like multiple chips on a PCB (RAM, SSDs, etc.) or where there’s a gap between the chip and the heatsink (e.g. something using the chassis as a heatsink).
There’s also thermal epoxy, which glues the heatsink to the chip, like with smaller cards that don’t expect to have the heatsink removed.
For a CPU or GPU, paste is still better.