One is economy, particularly if you prefer one-shot development, which brings about the advantage of consistency, though one could also make the case that a seasoned replenished developer also produces consistent results and is more economical, if you process a sufficient volume of film on a regular basis.

With developers that have a lot of sulfite, like D-76, you'll get higher acutance and sharper grain at higher dilutions. It's not as noticable at 1+1, but if you like that effect, try 1+3. Some people add sodium sulfite to developers like Rodinal to soften the grain a bit.