The rendering buffer is a vendor parameter used for GetMap and GetFeatureInfo.
It takes into account that the features are drawn with a style that makes them larger/bigger, in GetMap that will have the effect of increasing the queried area to catch the features that sit outside of it but whose symbolizer is big enough to get into it, and for GetFeatureInfo, it represents the search area
Normally the buffer is computed automatically using a static analysis of the style, but that won't work if the style has attribute dependencies of any kind (stroke width stored in an attribute, dynamic symbolizers, etc).
For those cases a &buffer=xxx param should be specified, but not all clients allow to setup that param in the request.
Allow the admin to specify a buffer hint on a per layer basis instead