When we require inputs to be confirmed, we can reliably check whether they are unspent. We can't reliably check this for unconfirmed inputs, because they could be valid but simply not in our mempool, in which case bitcoind would incorrectly consider them unspendable.
We want to reject unspendable inputs early to immediately fail the funding attempt, instead of waiting to detect the double-spend later.
When we require inputs to be confirmed, we can reliably check whether they are unspent. We can't reliably check this for unconfirmed inputs, because they could be valid but simply not in our mempool, in which case bitcoind would incorrectly consider them unspendable.
We want to reject unspendable inputs early to immediately fail the funding attempt, instead of waiting to detect the double-spend later.