They are location identifiers for a scenario where you have multiple P&Ls feeding into one balance sheet. In that case each set of P&L codes will have the same suffix all the way down - eg 9999.100 for the first P&L, 9999.200 for the second P&L and so on. You'll have a separate P&L for each store but one balance sheet and one trial balance.
I don't think there's any reason to use them for any other purpose. You can if you want - might be helpful for exporting to Excel for instance - but M3 doesn't use them except as shown above. Also, since Financial Designer keys off account names, not account numbers, it can get a bit confusing if you're not careful.
