No, Arial is not used just because of its ubiquity (at least not by publishing/communications professionals).
I remember while doing a lot of publishing work in the early to mid 2000s that we were instructed to use Arial or Verdana (both Sans Serif fonts) when authoring content to be read online. However, if we were publishing content for printed materials (e.g. books, newspapers, magazines, etc) we were supposed to use Serif fonts instead.
Dr. Ralph Wilson did some interesting research into the usage of fonts.
When people were surveyed on whether Times New Roman (a Serif font) or Arial (a Sans Serif font) was easier to read on a computer screen, the results were approximately 2:1 that Arial was much more readable.
These findings resulted in a new survey to compare Verdana and Arial, two different Sans Serif fonts. Interestingly, the results showed that at 12 pt Arial was easier to read, but when the sizes were reduced by two points, Verdana was much easier to read. Decreasing the font size further only increased the gap, so much so that at 9 pt Verdana was picked by 74% of those surveyed as being easier to read.
Going by the results in this survey, if your subtitles are for online purposes and they're 12pt or higher, then you should use Arial. On the other hand, if your subtitles are 10pt or smaller, you should use Verdana.
Of course, this changes again if you're talking about printed material, in which case I've always been led to believe one should use Serif fonts like Times New Roman.
If you're wanting to pick a font (rather than just choose between Arial and Verdana), perhaps the most important factors are to consider your audience and your content. For example, a serious medical report would not come across well if written in Comic Sans, although a kids party invite might look great using Comic Sans.
Other useful resources for you to consider are:
I hope this helps you somewhat. Good luck!