If a patron pledges one creator $1 a month, a huge amount of that would be lost to processing charges – either Patreon cuts deep into the creator's share, or they lose money. But – with charge bundling – if a patron pledges 15 creators each $1 a month, the fee stops being so bad, because the $0.30 is only charged once. The fee works out to 2.9% (3¢) plus 2¢ per creator, so each creator's fee on their $1 is only $0.05.
Fundamentally, charge bundling is what made Patreon able – in so far as it was able, and it's not actually clear they were able – to offer what really was the first functional micropayment system on the internet.