Why do I need a mailer for WP Mail SMTP?

Probably this is not the right place to ask this question, but it was raised by the WordPress plugin WP Mail SMTP by WPFORM.
So after installation I was asked which mailer do I use?
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Again, from my Joomla mindset, I expect the plugin will simply let me input my Email’s SMTP info, be it Gmail or Yahoo or some email provided by cPanel. But why do I need to choose a mailer here? Let’s say I use an email provided by some share hosted cPanel and it is something like example@my_other_site.com, can I still choose Gmail as my mailer´╝č If I use a Gmail account, would Gmail be the better choice for mailer despite it says Gmail has a API call limit?

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Or I should go with SMTP.com even though I am using a Gmail account? So I’ve got a little confused here. Can someone explain?