A blockchain is a chain of blocks. For most database applications, it makes sense to add transactions to the database immediately rather than wait to group them into blocks. Why then is the sacrifice of blocking transactions (from the perspective of a database user), a good design decision for the overall system?

Blocks are necessitated by mining, whereby an entity is awarded the ability to add a block and determine which transactions it contains (as long as all transactions are valid according to the protocol). The benefit of mining is that it enables distributed consensus and immutability. Proof of work and proof of stake are two different methods for determining who will mine blocks, but both strive to create decentralization and immutability.

Thus for what applications do decentralization and immutability matter? They are important for applications where there may be censorship or failure of centralized entities. In addition, decentralization can help with system reliability and robustness. Hence, money is a great application for blockchains. Arguably, social media is also a good applications as we've lately seen more censorship of social media.

Finally, public blockchains are open & permissionless. They have the advantage of allowing anyone to innovate. Whereas you need Facebook's permission to build an application on top of it, anyone can build a Steem application (like Steem Bounty and many others). Thus the pace of development is much more rapid for systems built on a blockchain, despite the underlying inefficiency of the data structure.