What is high RAM ($ff80-$fffe) useful for ? If used for the GB stack, does it provide better performance ?

No better performance can be obtained by using high RAM for the GB stack. But, if you are coding in assembly language, some opcodes are optimized (and hence operate faster) when using high RAM for data storage. For high performance code it is actually better to put the GB stack in lower RAM and use high RAM for data storage instead.

Sprite DMA code must be performed in high RAM because all other memory is disabled during Sprite DMA.