Why does the human body need buffers?
To maintain pH homeostasis.
pH tolerances vary by body system, but in every single case it's incredibly important to maintain it due to its undesirable effects, like the denaturing of proteins.
A buffer, which contains an acid and its conjugate base or a base and its conjugate acid, is capable of offsetting the introduction of an undesirable quantity of either an acid or base into the body. The buffer can become overwhelmed and become no longer effective at neutralizing either the acid or the base it has set out to offset. The terms acidosis and alkalosis are used to describe situations when the body has too much acid or too much of a base within it. Both of these can be fatal.