I didn't get the idea of letting the Tyrell's army into King's Landing but all the while holding off the Lannister one of Kevan. What does the Tyrell have to win if they're the only one confronting the sparrows? From Cersei's perspective I do get that she hopes it backfires on the Tyrell one way or another, probably the common people won't support them anymore weakening them and their claim to power.