As someone else mentioned, the practical help that anyone can actually give in one day is limited to say the least. In fact it might not be an exaggeration to say that an unskilled person who is there for one day is more likely to be a hindrance than a help.
So what is going on?
My theory is that they've noticed that people who visit the site tend to donate toward the project. This makes sense from the standpoint of Cialdini's persuasion principle "commitment and consistency". In other words, someone who makes a commitment toward someone is likely to follow it up with other acts consistent with that commitment. So someone who has gone to the effort of making a trip all the way to Chelmsford for the day, is also likely to put their hand in their pocket and put £20 or £50 toward the project. Whereas someone who never visits might not feel the need contribute anything.
So encouraging visits might not be about the help but about boosting donations.
Someone else mentioned it might be about building solidarity and morale among JWs. But frankly I don't think WT even care much about that at the moment. They are so concerned about gathering funds that they're closing KHs and merging congregations with no regard for longstanding communities, friendships or morale.
