If you have the opportunity, it’s better to pay a few euros (it’s not much for us) and you have peace of mind. It’s not about normal light rain, it’s about heavy rain, preferably when the soil cannot absorb any water.
It always depends on the local conditions. With our Märkisch sandy soil, this situation only occurs every few years, then the barrel simply overflows and part of the garden is flooded. But it’s gone again after a few hours. It’s certainly different with clay soil or rock.