Why would someone use EARN instead of going straight to CeFi/DeFi?

It'll always depend on said individual/institution's investment goals, but there are some arguments which are more compelling depending on your ultimate goal.

1. Security: Especially for high net-worth individuals/institutions, this is very important. Leaving all your funds in just one provider means one single point of failure; this can lead to catastrophic loss of all of your principal in case that one provider fails, or a DeFi protocol is hacked. 
2. Complexity: if said individual/institution would like to hedge their investment by diversifying across different CeFi providers/DeFi protocols, they'd have to individually manage each one, constantly. Moreover, this includes keeping not just stablecoins, but native token reserves, perhaps across multiple blockchains (gaining exposure to currency fluctuation risk) for transactions in and out, rebalances, etc. Not to mention dealing with multiple wallets (and keeping them secure).
3. Flexibility: A single provider of earnings can (and oftentimes will) change rules/conditions/rates overnight. If all your funds are deposited with such a provider, you have two choices: take it, or leave. If you have accounts with multiple ones directly, you can trigger a costly and burdensome rebalance. 
4. Liquidity: (Granted, this doesn't apply to some providers - or even most, when it comes to DeFi). When depositing with some providers, you may have time locks, slashing penalties, and your tokens may be illiquid for a set period - either of your choice or as a precondition. Although this is less common than in the traditional system, it still exists.

Now, let's see how Public Mint EARN tackles each of these:

1. Security: The first risk mitigation strategy is diversification. Funds can - and are - distributed across multiple providers/protocols. There is no single point of failure. Even in case one of these fails or is hacked, only part of the principal is at risk - and even so, EARN provisions for another layer of risk mitigation: insurance.

1a. Insurance: Initially, we're provisioning 10% of the total value locked (TVL) as an insurance fund, However, our roadmap includes actually integrating with a few Insurtech partners who cover both DeFi and CeFi, and allocate a portion of the insurance fund to them;

2. Simplicity: Pubic Mint is focused on simplicity and familiarity. EARN does away with all crypto complexity and hides it under a simple user experience. First, the Public Mint blockchain runs on fiat (USD). Fees, where they exist, are minimal and paid in fiat, doing away with the complexity and volatility risk of exposure to native crypto tokens. You don't need an exchange - a bank transfer is enough to move your dollars on-chain. You'll only ever need one wallet (but nothing stops you from creating new ones). Finally, any rebasing between earnings providers happens automatically under the hood. 
3. Flexibility: In case of a sudden change in conditions/rates at one or more earnings providers, the community of MINT holders will have full control over the decision of what to do next (after a transition period described in the lite paper). They can decide if and how to rebalance funds, add or remove providers, and change other parameters of EARN itself. Public Mint will execute on governance decisions - no costly/burdensome manual rebalances for users. It's worth noting that, by holding MINT and taking part in the governance, you'll also earn part of the 15% fee that you mention.
4. Liquidity: USD deposited on EARN is returned to users in the form of USD+, which is a fully liquid yield-bearing token. There's no time lock nor restriction on transferring USD+. You can hold it and watch your USD+ grow (and then redeem it), or you can use it to pay for services that accept it in the network, as it's fully liquid.

Last, Public Mint has more features than just EARN: It's a whole ecosystem. It's an open blockchain where new fiat-based financial products can be built. A DApp can be built that uses USD+ for other means, perhaps creating more value. It's an open-ended system.