Generally, beer doesn't age well. Except for high alcohol ones (which may be interesting to let spend some time in the cellar), beer start to significantly change from the very moment it gets bottled. Hops are the first to fade away, while temperature and light exposure can lead to premature oxidation. After "best before" date, beer won't be bad as such, but definitely not at its best anymore
The BB date is a choice of the brewer, AFAIK, and usually depends on how fresh they want you to drink their creation...