I've been wondering this too. I keep meaning to ask Charlie, but he takes about a fortnight to reply to PMs, and frankly, I can't be arsed.
From what I've seen of his 'spiel', promising to purchase a search pack immediately shows the vendor that you're deadly serious about the purchase; it also speeds up the process by about a month because conveyancers are slow, and whilst they will eventually do this, who knows how long it will take? They have to go through money laundering hoops and suchlike before they start spending your money.
In reality, I reckon this 'bargaining chip' is only really useful in a very vibrant market where a vendor is likely to receive multiple offers; in a sluggish market, I feel as though it would seem like you are just too keen if you offer this, frankly. Perhaps it's useful if you're trying a really cheeky low offer? Idk.
Also, remember Charlie gets paid money every time someone uses his brokers, conveyancers, and orders a search pack. So there's a financial incentive for him, too.
I don't exactly know what's in a search pack, but I imagine it covers possible problems with the local area: mining, flooding, massive housing developments due to take place up the road, etc - so it won't necessarily be specific to one individual home.
I did have something from a sale that fell through that covered all of the above. So I think that might be what it is.
Edit: I've also always wondered if the conveyancer would insist on their own search pack or accept what Charlie's firm provides
And if this information expires and you might just need to buy it again anyway if the purchase takes 3, 6+ months because stuff like planning applications become dated very quickly
Answers on a postcode pls