Why should I use the esc_url?

If you check the documentation on Data Validation it has following to say about the function:

Always use esc_url when sanitizing URLs (in text nodes, attribute nodes or anywhere else). Rejects URLs that do not have one of the provided whitelisted protocols […], eliminates invalid characters, and removes dangerous characters.

There you have it — practical security benefit. Valid protocol, no murky characters.

The answer about necessity is firmly yes. Escaping output is the most basic security practice.

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