IMO they wanted Pickett to be the starter.
I don't know if this was the case. I think they liked Pickett because he was cheap to have on the roster and cheap to acquire. Plus he started a ton of games in his career and that means something to teams. What changed everything was drafting two quarterbacks. I don't think they expected to do that.
If Pickett was intended to be the starter, then why sign Flacco?
Obviously, I don't have anything definitive to back this up but it is my guess that the Browns wanted Pickett to be QB2 while they tried to work out a deal with ATL for Cousins. Then they would have had him, Pickett to back him up and then potentially a draft pick if one they liked fell to them or, if one is inclined to think based on where Gabriel was selected, reach for one. Once Cousins wasn't going to happen, they immediately pivoted to Flacco.
But that's just me.