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The shop that kept the ticket
Retailer fraud is rare, occasionally spectacular, and the reason you sign first.
There have been documented cases of shop staff telling a customer their ticket was a small winner or a loser, keeping it, and claiming the real prize themselves. Several jurisdictions have run sting operations and found it happening at measurable rates.
It is not common, and modern terminals make it harder, many now display the result to the customer on their own screen. But it is the single best argument for the two habits in this article that cost nothing.
Sign the back before you hand it over. Check it yourself in the app first. An unsigned ticket belongs to whoever is holding it, and that is not a figure of speech.
Lotteries now run their own integrity testing for exactly this, sending shoppers with known winners into randomly chosen retailers. The results are published in some jurisdictions and are worth reading if you buy tickets in person, because the failure rate is never zero.