Recommendations for placing the robber are in the next step; they follow regardless of whether you move it on a roll of 7 or by playing a knight.
Knight: Knights are powerful - they protect your most valuable tiles, block other player's most valuable tiles, give you a free card (so you effectively only spent two cards, and denied an opponent one!), and get you in the race for Largest Army. If the robber is on one of your tiles, it's nearly always correct to play a knight before rolling unless you have 7 cards (robbing someone will give you 8 cards, so if you then roll a seven, you'll lose half your cards). Recommendations for placing the robber are in the next step; they follow regardless of whether you move it on a roll of 7 or by playing a knight.