have a look at this very good explanation from a user on the volocityreviews forum:
"There are some advantages to having a L3 etherchannel rather than having equal cost paths seen by L3 as two separate links. For
instance, imagine that one of the links starts flapping. With an etherchannel, the routing protocol will not see that at all since the port-channel will always stay up. With independent equal-cost links rather than an etherchannel, the flapping links will cause routing updates and the attendant cpu activity trying to converge etc. In such
a case, the etherchannel would give to load sharing and link redundancy with less overhead than if the links were treated as independent links.
Think of the situation where the component links of an etherchannel on a switch are connected to a interfaces on a router. The router will have to treat those interfaces as a bundle since that is how the switch
is treating them. Hence the router too will have to support etherchannel and have a corresponding interface that represents the
bundle of component links. Since this is a router, that etherchannel interface will have an IP address and operate at layer3. And that is exactly what a layer3 etherchannel is."