At a Catch weight, at 147, with no belts on the line, the fight as a whole is not the same. Has to be at 140 for all of the belts. Taylor's own words at the weigh for Catterall were "I'm bigger, faster and stronger...tomorrow I'm gonna be a beast". He can make 140 again. He can't move up to 147 with this hanging over his head. What better motivation than to train to prove your (many) doubters wrong and aim to clearly beat a guy he really was expected to beat handily the first time round.
Might as well. I don't know who else Josh could face at 147. Most the top welters are either with PBC, or on the DAZN side.