Great question — answering as the UphillTech team:
This is a nice easy one - the reason working titles exist is to help the on-set production team deal with the public on public sets - if they think a big star is going to be about they often loiter longer, all day even or worse still they shout out stuff when they see their favourite star appear. Whereas if they say it's a dull sounding film or even advert/commercial then people just walk on by and disruption is minimised/avoided. The rise of social media hasn't helped with this of course, once news is out that movie star X filming just across town the local area can get very busy. The use of the WT on the slate is for just the same reason, if the public were told a detergent...
Full answer by Chopper3: https://movies.stackexchange.com/a/118276 (CC BY-SA 4.0).
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