Here's why:
* Consistency: Capitalizing the first word of a subtitle, regardless of what it is, helps maintain consistency in your writing and makes the subtitle visually distinct from the main title.
* Clarity: It visually signals that a new section or part of your text is starting.
* Professionalism: It's a standard practice in academic writing, journalism, and most other forms of written communication.
Example:
Main Title: The Importance of Sleep
Subtitle: An Essential Guide to Restful Nights
Note: There are some stylistic exceptions, like when a subtitle is a single word, but the rule of capitalizing the first word generally applies.