For method robustness and reproducibility, it is important that a consistent pH is maintained. Any buffer used must therefore have sufficient capacity to resist changes of pH upon dilution and/or the addition of small quantities of acids or bases.
If buffer molarity is too low then this capacity will be compromised. However, if buffer molarity is too high then mobile phase viscosity will be unnecessarily raised and there will be an increasing risk of buffer salts precipitating.
For the majority of reversed phase methods, buffers in the concentration range of 10 -20 mM are suitable.