When, if ever, would you need to use: example.com/en

[CosiCrawley]

Is there any reason to have /en on your website if your website is only in English? Or is it worth having in case you are preparing to translate into other languages?

And is there any advantage to being:

en.example.com over example.com/en

Thanks

[TimHolmes]

As per Mark, if you are only planning on using one sole language then I would not bother creating a sub directory for your site to reside in when it would sit best in the root.

If however, eventually you plan to branch out again and become a more global operator I would keep your master site in the root and then branch out for supplementary languages e.g .com/fr or .com/de depending upon what you have planned for the future.

Again as per Mark, sub folders are my preferred option rather than sub domains, but both have their pros and cons.

If you do indeed look to branch out also look into serving alternate language set up and definitely check out this article from Search Engine Land on multilingual sites. a great resource.

[Mustansar]

Well as Mark rightly said you only use language initials if your site is multi language. Moreover, for English site you never use such initials as it will not make any difference though kind of misleading by giving impression to visitors you may have this site in other languages and they are browsing English version of that. If visitor want to access site in other language then there is no option.
I would urge to you refrain of doing that unless you have multi language site.

[Mark.]

Hi,
You would only use this for a website with multiple languages. So I can’t think of any reason to use /en/ for an English only site.

About the subfolders vs subdomains, there is a lot of info about this topic online. I would use subfolder instead of subdomains so you have more benefit from the root domain. Google sees subdomains like en.domain.com as a totally new domain. This way you need to build authority and rank for each individual subdomain. Here is some information from Google Webmasters: https://support.google.com/webmasters/answer/182192?hl=en