They don't allow you to hunt any later after shooting light, but help greatly to determine shot placement and to plan your tracking. Especially with bears, and anything at last shooting light it would help benefit recovery greatly knowing exactly where the arrow hit.
Lighted knocks are not currently legal in Idaho. There was a proposal made in 2011, but it was not approved by the Commission. At this time there is no lighted knock proposal for consideration as a non-biological rule for 2012.