Help me understand Dencun Upgrade

Hi community!

I do understand the point of proto-danksharding, or at least, what it is trying to achieve.

Here is a paragraph quoted from thedefiant.io :

Unlike calldata, blobs do not compete with Ethereum transactions for gas, improving data availability. Blob data is also deleted after two weeks, whereas calldata is stored indefinitely on the Ethereum blockchain.

• How do Blobs actually improve data availability?
• Particularly, how does deleting data (blob data is deleted after 2 weeks) improve data availability? It seems counter-intuitive to me.

Thanks for your help!