There are 17 Sickles in a Galleon, and 29 Knuts in a Sickle. This seems very impractical, as both numbers are primes.
It might be an out-of-universe parody of how old muggle money was divided before it was made decimal, but that old system at least made some sense: the divisions of 12, (or 20) meant that they had a lot of divisors, so you could easily divide it in 2, 3, 4 (or 5) equal parts.
Is there a reason why wizard money is divided in such a way?
The Wizarding coinage system is almost certainly based off (or possibly spoofing) pre-decimalization British coinage.
More specifically, the exact numbers of Galleon/Sickle/Knuts is probably to do with the weirdness relating to the gold Guinea coin in the late 17th century, where the fluctuating value of gold relative to silver resulted in the "1 pound sterling" Guinna coin actually being worth weird (and variable) numbers of shillings and pennies. Presumably, when the International Statute of Wizarding Secrecy came into play in 1692 (the aforementioned late 17th century), they just locked in whatever oddball divisions were current at the time for their own new Wizarding currency and those persisted for the next 300 years, without going through the Recoinage of 1816 or decimalisation in 1972, because Wizards are just a little bit traditional about things.
I think it's for humour, and to exaggerate the "old timey / old fashioned" nature of the magical world. After all, in England, old or old-fashioned people think it makes sense to divide things into 12s or 20s, and not to use the same multiplier for all of them. Old books use even more complicated things like guineas that are strange combinations of other amounts. So the wizarding world, which is even more old fashioned and old timey, would naturally use even weirder numbers and combinations of numbers.
