Why does a solution of a weak base and its conjugate acid act as a better buffer than does a solution?

Hello!

A solution of a weak base and its conjugate acid act as a better buffer than does a solution of a weak base alone because A solution of a weak base alone has no acid present to absorb added base. 

If an acid (In this case HCl) is added to a buffer or a weak base solution, the following reaction happens:

HCl + A⁻ → HA + Cl⁻

In this way, the addition of acid is neutralized by the base.

If a base is added (In this case NaOH), only a solution of a weak base and its conjugate acid can react in the following way:

NaOH + HA → NaA + H₂O

So, a solution of only a weak base can resist the addition of acids but not bases, so it isn't a good buffer. 

Have a nice day!