Best answer by BobTheElectrician
You get a 10% credit, so if your plan is $15 a month and your auto top-up if $15, your balance increases by $1.50 per month instead of being zero all the time. At some point 10 months down the road, you can cancel the auto top up and get a "free" month from your blance, or use the balance towards a booster purchase.
Incidentally, on the day of your auto top-up, if you check your account, you will sometimes get a nonesense message that your account has expired even though you have a positive balance way more than enough to cover the renewal. It should disappear by the folllowing day, but it's a needless stoopid PITA that a major company should not be inflicting on faithful customers.
By the way, all of these amounts are net of tax, so your auto top up will be $15 + appropriate taxes for your locale, probably $17.25 or so.