Delegated harvesting has many benefits for users and most importantly, for the NEM ecosystem.
First, delegated harvesting allows NEM to use a tiered architecture. The first tier is supernodes. Supernodes do the heavy lifting of creating blocks and confirming transactions. The next tier is light clients, such as the NanoWallet. NanoWallet users can collect harvesting rewards without downloading the entire blockchain, and continue to collect rewards even while their computers are off.
Another advantage of delegated harvesting is that it enables a high proportion of users to participate in the Proof of Importance (POI) system. The POI system rewards users for keeping a certain amount of currency in their wallet over time, but also for making frequent transactions. Both of these behaviors are good for the ecosystem, and delegated harvesting is the main reason users want to increase their POI scores.
Harvesters collect transaction fees from the blocks they harvest, and NEM does not take a cut for themselves. Harvesters keep 100% of their catch.
One more benefit of NEM's delegated harvesting feature is that light clients use a public key, which means a harvester's private key is never exposed. Even if the Supernode became compromised it would pose no danger to the clients harvesting with it.
Finally, delegated harvesting is a great example of the spirit in which NEM was founded. Instead of rewarding those with a huge bankroll to buy hardware farms or hoard coins, delegated harvesting allows more people to participate in the reward system. This is part of NEM's culture of fairness and inclusiveness, where anyone can have a positive impact.