Experience has taught the community that it's effectively impossible to keep intruders out. It is a question of when, not if, somebody will gain access to a your password database.
Doesn't matter whether you are a random blog or a multi-billion dollar government department, you need to assume that somebody will one day gain access. And quite often, they will have enough access to read the database but not enough access to, for example, insert a man-in-the-middle which grabs plaintext passwords as they're used to authenticate someone.
For example, they might not hack your primary server, they might only hack a server that contains backup copies of your database.
Also, most organisations have many employees. An employee doesn't need to hack your network to view the database, they might already have unfettered access (especially if they're an engineer or sysadmin). There are many reasons why it is a bad idea for your employees to know customer passwords.
Even if your website is completely worthless and it wouldn't matter if somebody hacks into it. The username and password used to log into your website is often exactly the same as the one used to log into other much more important services.
For example, somebody might write a bot that attempts to log into Apple's iTunes store with every username/password in your database, and if successful it starts purchasing things through the store. This attack might be successful for as many as 10% of the users in your database, and many of those users will never even notice they've been billed $4.99. This is not a theoretical attack, it happens all day long every day and attempts to stop it do not always succeed.
EDIT: And in the comments, @emory made another point I forgot: somebody might file a subpoena or use some other legal process to gain access to the database, allowing them to see plaintext passwords unless you have a good hash. Note it's not just law enforcement who can do this, any private lawyer with a strong legal case against you can get access to your database.