What's the point of ETHLend? I am trying to understand what the idea behing a site like ETHLend is.

IIUC, I can borrow coins for an interest, the same way I could borrow a traditional currency from a bank. That part is easy to understand.

But - again IIUC - they require that I block the collateral for the load - where they will only accept cryptocurrencies - upfront through the smart contract.

Translating to traditional terms, this would be as if the bank says:

Transfer 10.000 USD to us. Then we will borrow you 10.000 EUR which you can pay back in installments plus interest. After you will have paid your credit back in full, you will also get your 10.000 EUR back, otherwise we will just keep them.

What do I miss?
Your assessment is correct.

But think of it this way: to get a mortgage, you put up your house as collateral, where value(house)>value(mortgage). Same idea here.

Some response to your concern in their FAQ here


  Why would I borrow when I can just sell?
  
  
    There are a variety of reasons you may want to borrow and unlock the liquidity of your tokens rather than selling, some include:
    
    Borrowing Ether to participate in different ICOs,
     Purchasing other tokens from the exchange for different investment strategies without the need to sell tokens,
    Lending altcoins and tokens for short selling or hedging,
     Financing mining expansions,
     Finance for blockchain startups,
     Buying market dips.
    
    This list is expanded even further when we include USD Pegged Etheruem loans:
    
    Holding more ETH in a bullish market,
    Purchasing assets with $ while keeping your crypto portfolio safe,
    
    Future use-cases could include tokenisation of property, shares, intellectual property, and other physical objects (Cars, boats, power plants, art, etc)