Question

Why do buffers resist pH change?

Answer 1

Because they introduce a means for the solvent to sop up excess `H_3O^+` and `HO^-`, and resist gross change in `pH`.

Explanation:

This equation is well-known:

`pH=pK_a+log_10{[[A^-]]/[[HA]}}`

Thus given quantities of acid and conjugate base, added `H_3O^+` or `HO^-` reacts with the base or the acid, and `pH` change is moderated by the `log_10` term.

When a weak acid is titrated by a strong base, or a strong acid by a weak base, a buffer is formed initially, and this region of the graph of `pH` versus volume of titrant is known as the buffer region.

Biological systems are extensively buffered, to reduce gross changes in `pH`.

For a fuller treatment, see this old answer.