Casual gets your bot exposed to lower skilled MMR(?) players. Thus your bot might win a little longer. Plus some of us want to test new decks against players but not burn up the ranking. Completeness can be another reason. WIMM
Hop into a Casual game, select the deck you want, and, get into the game, once the mulligan animation is over (I.E. all cards are static) press start with all three buttons unticked, it'll log "Starting an unranked PVP game" and it'll continue to play casual. If you press go before the mulligan animation is over, the game will crash.