Some guys will flush condoms down the toilet as a discrete and convenient method of disposal.
Some guys or men will find it less embarrassing to dispose of the condom in the toilet and flush it down instead of leaving it in the trash can.
However flushing condoms is not a good idea as it can potentially lead to a clogged toilet or clogged sewer line if done enough.
Flushing one condom every now and then may not cause any issues but doing it too often can lead to clogs.
It's best to throw the condom in the trash instead of flushing them.
Condoms don't degrade in water like toilet paper does or human waste.
The condoms when flushed also won't break down when caught on a screen or on a bump in a compromised pipe.
If the condom happens to get snagged on something it will create a built up blockage once enough stuff gets caught on it.
Same reason you don't flush tampons, pads or wipes etc.