By default, WordPress uses the wp_mail function to send emails via PHP mail (including any Avada notification emails). So your site is sending emails via the same server that your site is hosted on, which is almost certainly not optimized for sending emails. It’s very simple, and it sort of works (mostly), but it lacks proper authentication, which is why emails that you send via the PHP mail method often end up in spam, or even worse, just get outright blocked by some email providers.
SMTP, on the other hand, tells your WordPress site to send emails via a properly-configured SMTP server, rather than using your site’s server. Instead, your website sends the email to the SMTP server, which then processes the email and sends it on to the user’s inbox. Because the SMTP server is configured to handle sending emails and has the proper authentication in place, your emails are much more likely to make it to recipients’ inboxes.
As well as this, some web hosts don’t even offer email as part of their hosting package. For example, neither Kinsta nor Flywheel offers email hosting, and so in these cases you would absolutely need an SMTP plugin.
Thirdly, deliverability of emails is of vital importance to a business, and instead of relying on a hit and miss default solution, you can use a third-party SMTP server that’s 100% dedicated to sending emails.