Phosphate buffers uses phosphate, sodium or potassium for example, to maintain a steady pH. As noted, plants use phosphate as a macro nutrient so any buffering effect will be short lived as the plants will consume it.
It was a common belief that phosphates can lead to algae, and in a highly lit environment like a planted tank it would cause algae issues. That has been proven false. However, there is little reason to tamper with the water's pH unless you're at the extreme ranges so why waste money trying to fix nothing?