one benefit of this that i can think of is that we can send these immortals to go for space explorations. normal humans will just die along the way since the universe is so big.

another one is like what mentioned by DeniseLau, we can gain continuity of knowledge. imagine if geniuses like einstein or nikola tesla is still alive and continued with their research.

to make it fair i think anybody who is given immortality should be disallowed/disabled to reproduce. that'll help balance things up a bit i think.